libvirt_configure: persist cmd qemu-sanlock.conf

On a ovirt-node if executing vdsm-tool configure --force
with libvirt sanlock enabled it will set /etc/libvirt/qemu-sanlock.conf
but won't use persist command so next reboot the changes will be lost
and vdsm-tool will complain about libvirt not configured during vdsm start.

Change-Id: I299f5cb7af4d2e09208835aeda28ec1f3c3def7d
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/25968
Reviewed-by: Fabian Deutsch <fabiand@fedoraproject.org>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>