spec: Require qemu-img-rhev in EL6

In EL qemu-kvm does not support live snapshots. In order to support
them, qemu-kvm-rhev is required. qemu-img-rhev must be used when
qemu-kvm-rhev is used.
These packages are now available in the ovirt-releases repository.

Change-Id: I22c5f4a6bd99734219b80d177642456382cfb675
Related-To: https://bugzilla.redhat.com/1127763
Requires: http://gerrit.ovirt.org/31214
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>