Make compute_api confirm/revert resize use objects

This makes the compute_api confirm and revert_resize calls use
and return Migration objects for its work.  This also adds the
cells RPC methods needed to support child cells re-calling the
compute API with objects.

Related to blueprint compute-api-objects

Change-Id: I26adc8d2fadc4a31cd0de94887e163040691ede1
Merge "Make compute_api confirm/revert resize use objects"