Moved the registration of lifecycle event handler in init_host()

The event handler has to be registered before the libvirt connection
is used for the first time.  Otherwise it doesn't register and events
such as shut down initiated from within the instance are not registered
in nova until a period task compares the content of hypervisor with
content of database.

Closes bug: #1297485

Change-Id: I70887bea343734a977f621fedc634b6fab0e4ad5
Merge "Moved the registration of lifecycle event handler in init_host()"