spec: finer-grained libvirt requirement

We do not have to pull in all libvirt drivers.

Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Change-Id: Idd7bb89d239a88012d9bcd0d30874a1d4993e20a
Reviewed-on: http://gerrit.ovirt.org/15760
Reviewed-by: Adam Litke <agl@us.ibm.com>
Tested-by: Mark Wu <wudxw@linux.vnet.ibm.com>