Parse backing file from qemu-image info output

I'm deplying thin clones of VM's using the qcow2 disk format.  One of the lines
of the qemu-image info command reports the filename of the base disk.

   $ qemu-img info /work/aantony/kvm/ajith-sl6-2.qcow2
   image: /work/aantony/kvm/ajith-sl6-2.qcow2
   file format: qcow2
   virtual size: 20G (21474836480 bytes)
   disk size: 1.0G
   cluster_size: 65536
   backing file: ./ajith-sl6-base-image.qcow2 (actual path: /work/aantony/kvm/./ajith-sl6-base-image.qcow2)

This last line is breaking the yaml parser:

   backing file: ./ajith-sl6-base-image.qcow2 (actual path: /work/aantony/kvm/./ajith-sl6-base-image.qcow2)

   ScannerError: mapping values are not allowed here
     in "<string>", line 6, column 51:
        ... l6-base-image.qcow2 (actual path: /work/aantony/kvm/./ajith-sl6- ...

This change aims to at least parse that line.

Results look like this:

  ----------
    ----------
    ajith-sl6-2:
        ----------
        cpu:
            1
        cputime:
            6383750000000
        disks:
            ----------
            hda:
                ----------
                backing file:
                    /work/aantony/kvm/./ajith-sl6-base-image.qcow2
                cluster_size:
                    65536
                disk size:
                    1.0G
                file:
                    /work/aantony/kvm/ajith-sl6-2.qcow2
                file format:
                    qcow2
                image:
                    /work/aantony/kvm/ajith-sl6-2.qcow2
                virtual size:
                    20G (21474836480 bytes)
     <...>
Merge pull request #7029 from ajithhub/base_disk_parse

Parse backing file from qemu-image info output