Stopping VMMs mid-migration seems buggy, a little unexplored

https://github.com/oxidecomputer/omicron/issues/8755 has a lot more context, in particular: https://github.com/oxidecomputer/omicron/issues/8755#issuecomment-5332927871 and https://github.com/oxidecomputer/omicron/issues/8755#issuecomment-5333712500 In summary: - Migrating instances can be stopped - We don't actually try to tell the target to stop - It's not clear to us propolis-server is doing the right thing when this happens - we may need Nexus to actually tell *both* VMMs to stop in this scenario.