Make xenapi use Instance object for host_maintenance_mode()

This makes xenapi grab an Instance object in host_maintenance_mode()
instead of calling into virtapi. This removes the one and only use
of instance_get_by_uuid() in nova/virt.

The existing tests for host_maintenance_mode() need no changes
since they just stub out the database call that eventually gets
called in the same way.

Related to blueprint unified-object-model

Change-Id: Iff4b5de1741653fad8c59e311341aeb3d0bb3539
Merge "Make xenapi use Instance object for host_maintenance_mode()"