spec: qemu-img is required during the tests

The tests use vdsm/qemuimg module, which checks where qemu-img is
located. We can probably mock it but it does not seem worth the effort.

Change-Id: I609904760c47c064ae24047da32e9e8879b1e476
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33507
Reviewed-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>