Introduce basic infrastructure for virtlockd daemon

Introduce basic infrastructure for virtlockd daemon

The virtlockd daemon will maintain locks on behalf of libvirtd.
There are two reasons for it to be separate

 - Avoid risk of other libvirtd threads accidentally
   releasing fcntl() locks by opening + closing a file
   that is locked
 - Ensure locks can be preserved across libvirtd restarts.
   virtlockd will need to be able to re-exec itself while
   maintaining locks. This is simpler to achieve if its
   sole job is maintaining locks

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>