Description of problem:

Note this requires that you have fixed bug 1134592 in libvirt first.

If you create a backing file that references an ssh server,
libvirt doesn't pass the required SSH_* variables through to
qemu so it cannot access the ssh agent.

$ qemu-img create -f qcow2 \
  -b 'json: { "file.driver":"ssh",
              "file.path":"/mnt/scratch/test1.img",
              "file.user":"rjones",
              "file.host":"onuma.home.annexia.org",
              "file.host_key_check":"no" }' test2.img

$ ~/d/libvirt/run guestfish -a test2.img run
[...]
Original error from libvirt: internal error: early end of file from monitor: possible problem:
2014-09-09T11:36:32.304954Z qemu-system-x86_64: -drive file=/tmp/test2.img,if=none,id=drive-scsi0-0-0-0,format=qcow2,cache=writeback: could not open disk image /tmp/test2.img: Could not open backing file: failed to connect to ssh-agent: no auth sock variable (libssh2 error code: -39)
 [code=1 domain=10]

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

libvirt from git (2014-09-09) + patches to fix bug 1134592.

How reproducible:

100%

Steps to Reproduce:
1. See above.

Related: bug 856619