domainEventRegisterAny called too often

Changeid I090765802bfe443440f16722bc7c43b6280fe56a fixes
bug #1240905 but introduced a bug whereby domainEventRegisterAny
is called on every call to _get_connection.

Conflicts:
        nova/virt/libvirt/driver.py

NOTE(mriedem): The conflict is due to the set_host_enabled
code not being in stable/havana and we don't want to backport
that for the threading fixes in this series.

Change-Id: Ifafc6a4c172b1521d62988ccbb3a2f51a95e323f
Fixes: bug #1250135
(cherry picked from commit 755f6194e29c8e3daf6fd64a59dc2be4c81b1f0d)
Merge "domainEventRegisterAny called too often" into stable/havana