Fix UnboundLocalError in libvirt.driver._close_callback

If for whatever reason _wrapped_conn is None or not equal to the conn
parameter passed to the _close_callback method, the local _error
variable won't be set when calling set_host_enabled and result in an
UnboundLocalError.

Partial-Bug: #1257644

Change-Id: I40855f22d5449f3b3646c90fe2d9a71560eb86c4
Merge "Fix UnboundLocalError in libvirt.driver._close_callback"