spec: do not require qemu-kvm-rhev on Fedora

qemu-kvm-rhev is required for oVirt's feature completeness. However it
is not shipped within Fedora. When Vdsm is built for Fedora, we cannot
require it. However, when built for oVirt and downstream RHEV, it is
simpler and nicer to have a strict hard requirement.

Change-Id: If272983362d5b0f11d0925c64315a046d3b144d0
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33367
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>