use qemu-img convert to copy sparse volumes when moving a vm

Previously all volumes were copied with dd, with sparse files this
would result in the destination volume being fully allocated.

Change-Id: I9eedaa41a64502e548e52c10742335abd05301d8
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=748386
Signed-off-by: Yeela Kaplan <ykaplan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/10979
Reviewed-by: Dan Kenigsberg <danken@redhat.com>