Compute: Catch binding failed exception while init host

While compute starts it will init all instances,
if an exception is raised from one instance
(e.g NovaException during plug_vifs), then the
compute process exits unexpectedly because of
this unhandled exception.
This commit changes the NovaException to more
appropriate VirtualInterfacePlugException and
catches it during init host, as well as the
instance is set to error state, with this change
the compute process can be started normally even
if this VirtualInterfacePlugException is raised.

Closes-bug: #1324041

Change-Id: Ia584dba66affb86787e3069df19bd17b89cb5c49
Merge "Compute: Catch binding failed exception while init host"