Fix power_state lookup in confirm_resize

Previously the compute manager's confirm_resize method was trying to get
the current power_state of the resized/migrated instance from the
hypervisor layer to account for a case where the user migrated from a
stopped instance and then powered it on via the hypervisor (below
openstack) to test it before confirming the migration.  This doesn't
work in the migrate case because the confirm_resize request is serviced
on the source compute node, and the instance no longer lives there so
_get_power_state results in an InstanceNotFound.  The vm_state would
then be set to ACTIVE even though the instance's power_state is still
SHUTDOWN.

This patch fixes this problem by just getting the instance power_state
from the database since the scenario of powering on the migrated
instance outside of openstack is not supported.

Fixes bug 1197514

Change-Id: If2ea83cd317aa8897b7b6bd6b43a2067e1074e33
Merge "Fix power_state lookup in confirm_resize"