qemuimg: Create qcow2 compat 0.10 images when converting existing images

Commit 1f7c3ac2c handled only creation of new images. This patch adds the
required -o compat=0.10 option when converting existing images.

_supports_qcow2_compat() accepts a command argument and invokes the
specific command to detect if the compat option is supported.

Change-Id: I733fdd8c0e50a98627794c19c61b1d9bf6d6d6b1
Bug-Url: https://bugzilla.redhat.com/1139707
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33349
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>