spec: fix name of qemu-img for el7

Change-Id: I7dea19ede39d68215f28a19edcbea52e062d022e
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23025
Reviewed-by: Ohad Basan <obasan@redhat.com>
Reviewed-by: Douglas Schilling Landgraf <dougsland@redhat.com>