Packaging: libvirt configuration detection should skip /dev/null

We disabled libvirt-sanlock in Ubuntu, so QLCONF is set to /dev/null,
and later we need to grep "${BY_VDSM_VERS}" from QLCONF to confirm
libvirt is configured. grep returns 1 for /dev/null but actually libvirt
is configured. This patch skip /dev/null when grep libvirt .conf files.

Change-Id: Ia0d8a976c023709068ca2f3691fab5cd59ae02ff
Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/20260
Reviewed-by: Dan Kenigsberg <danken@redhat.com>