make libvirt driver get_connection thread-safe

libvirt driver's get_connection is not thread safe in the
presence of a libvirtd restart during concurrent incoming
requests.

With existing code each will in turn call get_connection,
find the connection is broken, try to create new one, block
for a while and yield to the next thread to do the same.
You get as many connections as there are incoming requests
and only the last one is used finally. If enough are incoming
these connections can exhaust the client pool configured
for libvirtd.
One fix is to hold a lock while creating the connection.
Note that has_min_version calls _conn which calls get_connection
and thus the direct call to _has_min_version()

Also added the exception text if it fails to register an event
handler for lifecycle events.

Change-Id: I090765802bfe443440f16722bc7c43b6280fe56a
Fixes: bug #1240905
Merge "make libvirt driver get_connection thread-safe"