After exception on _recoverExistingVms thread we should try again

Currently we fail the process on exception and gets stuck on recovery.
This might be because libvirt exception during first connect. Anyhow, we
should continue try to recover to start serving vdsm requests.

Change-Id: I80745e04c7cce50e4cd8ce12628131b76d0659a7
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=991091
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17617
Reviewed-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>