Audit VMMigration controller code

[Omrigan]

VMMigration controller code is a bit neglected, as it didn't have any production use. We should fix it.

1. Assess the tech debt, fix what can be fixed fast.
2. Take a closer look at the state machine, see if there are any potentially problematic states.
   • E.g. Migrating + Scaling; Migrating + Deleting
3. Assess the possibility of merging VMMigration controller into the VM controller.
   • E. g. it makes state machine simpler to understand

[mikhail-sakhnov]

We once discussed auditing reasonability of having RECEIVE_MIGRATION=true after finishing the migration, more context here #1190 (comment)

[Omrigan]

One more thing to look at is failure to simultaneously transition VM and VMM states