Gather and send instance information on migration

[plotnick]

PR #1636 sends applicable firewall rules to a sled-agent on instance creation (see also issue #1017) and rule modifications, but not on instance migration. There are several other important pieces of information not currently migrated with instances; see the TODOs in instance_migrate::sim_instance_migrate. We should maybe have separate issues for each of them, but we'll list them here for now so they're all tracked:

• NICs
• Firewall rules
• Disks
• Cloud-init bytes

[gjcolombo]

The live migration saga rework in #2892 (and its sibling PRs) addressed this: instance start and instance migrate now use a common Nexus function (instance_ensure_registered) to send configuration to sleds (in this case the migration destination sled).