Update migration protocol to negotiate device version payloads

The Propolis library's migration framework supports composable versioned payloads through the PayloadOffer and PayloadOutput types in propolis::migrate. In this scheme, each chunk of migration data has a "kind" and a version, and each Propolis component (device or backend) can export one or more chunks. Because chunks are versioned independently, we can bump the version of a common subcomponent (e.g. the virtio state exported by all virtio devices) without having to re-version all of the independent versions that contain it.

Today the migration protocol has no affordance for the source and destination to agree on the component versions the source should send. (Note that it's not a given that the destination understands more versions than the source, since the migration may be a downgrade.) The protocol should include this, probably as part of the "preamble" phase that otherwise determines whether a migration from the source to the target will be accepted.

Start with the PayloadOffer and PayloadOutput types in propolis::migrate, then trace the migration preamble that decides whether a source and destination can proceed. Define how component version information is exchanged and selected, including downgrade cases. Done means the protocol can negotiate the versions of migration payloads before migration data is sent.