domainMonitor: Ensure that host id is released

Previously unexpected exception could cause the domain monitor thread to
exit without releasing the host id. Stale host id will prevent removal
of logical volume and the underlying devices, and prevent stopping of
sanlock, requiring a reboot to upgrade a host.

Now try finally block ensures that host id is is always released.

Change-Id: I99b8a11b055a671d180feff92500091b3e0e1287
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/30922
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>