[SANIBEL] Fix memory utilisation reporting (migration race condition)

Fixes a race condition in pool migration whereby the memory-actual field may be incorrectly set to a spuriously low value and never updated.

The change is described in commit f0f94eb.

This has been tested by adding a delay on the sender side of the migration prior to the destruction of the domain and the spoofing of a memory value on the sender side triggered by a FIST file.

It is likely this will also need fixing on trunk which will not just be a simple forward-port due to the `rrdd` and `xenopsd` disaggregation.