qemuimg: Handle new output format

The current qemu-img info parsing logic assumes that if the output is
long enough, the backing image will appear on line 6.  In newer qemu
versions, some additional information has been added to the output.  In
the case of no backing file, the additional lines will fail to parse.
Handle this by trying to parse line 6 as a backing file and ignoring
regex parse errors for this line only.

Sample output from qemu-2.0.0-rc1:

[alitke@lager qemu-2.0.0-rc1]$ ./qemu-img info base.img
image: base.img
file format: qcow2
virtual size: 1.0G (1073741824 bytes)
disk size: 196K
cluster_size: 65536
Format specific information:
    compat: 1.1
    lazy refcounts: false
[alitke@lager qemu-2.0.0-rc1]$ ./qemu-img info leaf.img
image: leaf.img
file format: qcow2
virtual size: 1.0G (1073741824 bytes)
disk size: 196K
cluster_size: 65536
backing file: base.img
Format specific information:
    compat: 1.1
    lazy refcounts: false

Change-Id: Ic229198ab7c2bb9743bdf8629416131186115431
Signed-off-by: Adam Litke <alitke@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27552
Reviewed-by: Dan Kenigsberg <danken@redhat.com>