spec: finer-grained libvirt requirement

Change-Id: I48e25ca9e307c2cde7f29b65c68ff6e122ec8f64
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/15761
Reviewed-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-by: Mark Wu <wudxw@linux.vnet.ibm.com>