Description of problem:

From:
https://kojipkgs.fedoraproject.org//work/tasks/8702/7748702/build.log

libguestfs: libvirt XML:\n<?xml version="1.0"?>\n<domain type="qemu" xmlns:qemu="http://libvirt.org/schemas/domain/qemu/1.0">\n  <name>guestfs-auco8tmn6tdw7qnd</name>\n  <memory unit="MiB">500</memory>\n  <currentMemory unit="MiB">500</currentMemory>\n  <vcpu>1</vcpu>\n  <clock offset="utc">\n    <timer name="rtc" tickpolicy="catchup"/>\n    <timer name="pit" tickpolicy="delay"/>\n    <timer name="hpet" present="no"/>\n  </clock>\n  <os>\n    <type>hvm</type>\n    <kernel>/builddir/build/BUILD/libguestfs-1.27.58/tmp/.guestfs-1000/appliance.d/kernel</kernel>\n    <initrd>/builddir/build/BUILD/libguestfs-1.27.58/tmp/.guestfs-1000/appliance.d/initrd</initrd>\n    <cmdline>panic=1 console=ttyS0 udevtimeout=6000 no_timer_check acpi=off printk.time=1 cgroup_disable=memory root=/dev/sdb selinux=0 guestfs_verbose=1 guestfs_network=1 TERM=vt100</cmdline>\n    <bios useserial="yes"/>\n  </os>\n  <on_reboot>destroy</on_reboot>\n  <devices>\n    <controller type="scsi" index="0" model="virtio-scsi"/>\n    <disk device="disk" type="file">\n      <source file="/builddir/build/BUILD/libguestfs-1.27.58/builder/phony-fedora.img"/>\n      <target dev="sda" bus="scsi"/>\n      <driver name="qemu" type="qcow2" cache="unsafe"/>\n      <address type="drive" controller="0" bus="0" target="0" unit="0"/>\n    </disk>\n    <disk type="file" device="disk">\n      <source file="/builddir/build/BUILD/libguestfs-1.27.58/tmp/libguestfsWFQJER/overlay1"/>\n      <target dev="sdb" bus="scsi"/>\n      <driver name="qemu" type="qcow2" cache="unsafe"/>\n      <address type="drive" controller="0" bus="0" target="1" unit="0"/>\n      <shareable/>\n    </disk>\n    <serial type="unix">\n      <source mode="connect" path="/builddir/build/BUILD/libguestfs-1.27.58/tmp/libguestfsWFQJER/console.sock"/>\n      <target port="0"/>\n    </serial>\n    <channel type="unix">\n      <source mode="connect" path="/builddir/build/BUILD/libguestfs-1.27.58/tmp/libguestfsWFQJER/guestfsd.sock"/>\n      <target type="virtio" name="org.libguestfs.channel.0"/>\n    </channel>\n    <interface type="bridge">\n      <source bridge="virbr0"/>\n      <model type="virtio"/>\n    </interface>\n  </devices>\n  <qemu:commandline>\n    <qemu:env name="TMPDIR" value="/builddir/build/BUILD/libguestfs-1.27.58/tmp"/>\n  </qemu:commandline>\n</domain>\n
[...]
libguestfs: [00178ms] launch libvirt guest
libguestfs: trace: launch = -1 (error)
virt-builder: error: libguestfs error: could not create appliance through 
libvirt.
Try running qemu directly without libvirt using this environment variable:
export LIBGUESTFS_BACKEND=direct
Original error from libvirt: failed to retrieve file descriptor for 
interface: Transport endpoint is not connected [code=38 domain=10]

Note the XML fragment we are using:

    <interface type="bridge">
      <source bridge="virbr0"/>
      <model type="virtio"/>
    </interface>

I'm prepared to believe this is some problem with the Koji
environment.  Perhaps virbr0 does not exist?

However if virbr0 does not exist then the error is a bit obscure
and should be made better.

Version-Release number of selected component (if applicable):

libvirt 1.2.8-6.fc21.x86_64

How reproducible:

100% (in Koji)

Steps to Reproduce:
1. Run a command such as:

  guestfish -a /dev/null --network run