This is fixed upstream.

Verified this bug with the following packages:
virt-v2v-0.8.3-1.el6.x86_64
libguestfs-1.7.17-26.el6.x86_64
libvirt-0.9.4-4.el6.x86_64

Steps:
[root@tzheng-rhevm Desktop]# virt-v2v -ic xen+ssh://10.66.72.123 -b rhevm -n virbr0 -os default xen-hvm-rhel4.8-i386
--network and --bridge are mutually exclusive
Usage:
     virt-v2v -i libvirtxml -os imported --network default guest-domain.xml

     virt-v2v -ic esx://esx.server/ -os imported --network default esx_guest

     virt-v2v -ic esx://esx.server/ \
              -o rhev -os rhev.nfs.storage:/export_domain --network rhevm \
              esx_guest

Options:
    -i input
        Specifies what input method to use to obtain the guest for
        conversion. The default is "libvirt". Supported options are:

        *libvirt*
            Guest argument is the name of a libvirt domain.

        *libvirtxml*
            Guest argument is the path to an XML file containing a libvirt
            domain.

    -ic URI
        Specifies the connection to use when using the libvirt input method.
        If omitted, this defaults to qemu:///system when virt-v2v runs as
        root, or qemu:///session when virt-v2v runs as a regular user.

        N.B. virt-v2v can currently automatically obtain guest storage from
        local libvirt connections, ESX connections, and connections over
        SSH. Other types of connection are not supported.

    -o method
        Specifies the output method. Supported output methods are:

        libvirt
            Create a libvirt guest. *-os* must specify a libvirt storage
            pool for the libvirt output method.

            Also see the *-oc* option.

        rhev
            Create a guest on a RHEV 'Export' storage domain, which can
            later be imported into RHEV using the UI. *-os* must specify the
            location of a RHEV export storage domain for the RHEV output
            method.

        If no output type is specified, it defaults to libvirt.

    -oc URI
        Specifies the libvirt connection to use to create the converted
        guest. If ommitted, this defaults to qemu:///system.

        N.B. virt-v2v must be able to write directly to storage described by
        this libvirt connection. This makes writing to a remote connection
        impractical at present.

    -os storage
        The output method dependent location where new storage will be
        created for the converted guest.

        For the *libvirt* output method, this must be the name of a storage
        pool.

        For the *rhev* output method, this specifies the NFS path to a RHEV
        Export storage domain. Note that the storage domain must have been
        previously initialised by RHEV. The domain must be in the format
        <host>:<path>, eg:

         rhev-storage.example.com:/rhev/export

        The nfs export must be mountable and writable by the machine running
        virt-v2v.

    -op pool
        See *-os* for the *libvirt* output method.

        DEPRECATED Use *-os* instead.

    -osd domain
        See *-os* for the *rhev* output method.

        DEPRECATED Use *-os* instead.

    -of format
        Specifies the on-disk format which will be used for the converted
        guest. Currently supported options are *raw* and *qcow2*. If not
        specified, the converted guest will use the same format as the
        source guest.

    -oa allocation
        Specifies whether the converted guest should be *sparse* or
        *preallocated*. If not specified, the converted guest will use the
        same allocation scheme as the source.

    -on outputname
        Rename the guest.

        If this option is not given, then the output name is the same as the
        input name.

    -f file | --config file
        Load a virt-v2v configuration from *file*. Multiple configuration
        files can be specified, which will be searched in the order they are
        specified on the command line. If no configuration is specified,
        defaults to /etc/virt-v2v.conf and /var/lib/virt-v2v/virt-v2v.db in
        that order.

        When overriding the default config file it is recommended that
        /var/lib/virt-v2v/virt-v2v.db is also specified, as it contains
        default configuration data required for conversions.

    -n network | --network network
        Map all guest bridges or networks which don't have a mapping in the
        configuration file to *network*.

        This option cannot be used in conjunction with *--bridge*.

    -b bridge | --bridge bridge
        Map all guest bridges or networks which don't have a mapping in the
        configuration file to *bridge*.

        This option cannot be used in conjunction with *--network*.

    -p profile | --profile profile
        Take default values for output method, output storage and network
        mappings from *profile* in the configuration file.

    --root=ask
    --root=single
    --root=first
    --root=/dev/sdX
        Choose the root filesystem to be converted.

        In the case where the virtual machine is dual-boot or multi-boot, or
        where the VM has other filesystems that look like operating systems,
        this option can be used to select the root filesystem (a.k.a. "C:
        drive" or "/") of the operating system that is to be converted. The
        Windows Recovery Console, certain attached DVD drives, and bugs in
        libguestfs inspection heuristics, can make a guest look like a
        multi-boot operating system.

        The default in virt-v2v ≤ 0.7.1 was *--root=single*, which causes
        virt-v2v to die if a multi-boot operating system is found.

        Since virt-v2v ≥ 0.7.2 the default is now *--root=ask*: If the VM is
        found to be multi-boot, then virt-v2v will stop and list the
        possible root filesystems and ask the user which to use. This
        requires that virt-v2v is run interactively.

        *--root=first* means to choose the first root device in the case of
        a multi-boot operating system. Since this is a heuristic, it may
        sometimes choose the wrong one.

        You can also name a specific root device, eg. *--root=/dev/sda2*
        would mean to use the second partition on the first hard drive. If
        the named root device does not exist or was not detected as a root
        device, then virt-v2v will fail.

        Note that there is a bug in grub which prevents it from successfully
        booting a multiboot system if VirtIO is enabled. Grub is only able
        to boot an operating system from the first VirtIO disk.
        Specifically, /boot must be on the first VirtIO disk, and it cannot
        chainload an OS which is not in the first VirtIO disk.

    --list-profiles
        Display a list of target profile names specified in the
        configuration file.

    --help
        Display brief help.

    --version
        Display version number and exit.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHSA-2011-1615.html