qemuimg: add backing file to convert

This patch adds the support for the backing file in the convert qemu-img
command.

Change-Id: I2d6f254f5750fa249809469ecc764a73c91914d4
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33354
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>