For a cross-cell resize, the equivalent of the "finish_resize"
method on the destination compute is going to call the
driver spawn() method rather than the finish_migration()
method and needs to pass through the power_on value, similar
to finish_migration, so that when resizing a stopped server
it remains stopped once it is resized.
The finish_migration method in the driver behaves very similar
to spawn so the semantics are the same.
This change updates the spawn method signature for all in-tree
compute drivers but only implements the logic for the libvirt
driver as that is the only driver (currently) which supports
cross-cell resize (note the can_connect_volume method is also
necessary for cross-cell resize implementation in the driver).
Part of blueprint cross-cell-resize
Change-Id: I6929c588dd2e0e805f2e30b2e30d29967469d756
a958dc5fcc