Description of problem:
- Currently, when trying to list available VMs to import from Xen using RHEV webadmin, if there is a block device disk configured on Xen, the list failed to load with the next vdsm.log error:
jsonrpc.Executor/5::ERROR::2016-08-08 15:59:30,265::v2v::918::root::(_add_disk_info) Error getting disk size
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/vdsm/v2v.py", line 915, in _add_disk_info
    vol = conn.storageVolLookupByPath(disk['alias'])
  File "/usr/lib64/python2.7/site-packages/libvirt.py", line 4596, in storageVolLookupByPath
    if ret is None:raise libvirtError('virStorageVolLookupByPath() failed', conn=self)
libvirtError: cannot read dir '/dev/sdb': Not a directory

- According to virt-v2v wiki, If the guest disks are located on a host block device, then the conversion will fail. 
see http://libguestfs.org/virt-v2v.1.html#xen-or-ssh-conversions-from-block-devices for a workaround.

- It would be nice to have an ability to import VM with block device from Xen using virt-v2v workaround.

Version-Release number of selected component (if applicable):
rhevm-4.0.2.4-0.1.el7ev

How reproducible:
100%

Steps to Reproduce:
1. Add Block device to Xen
2. Using webadmin import dialog, enter Xen details and click "load" button.
3.

Actual results:
list failed to load.

Expected results:
using virt-v2v workaround, it should be possible to list source VMs and import them.

Additional info:
vdsm.log attached (issue occured at: 2016-08-08 15:59:30,265)