Instance (re)creation from migration payload

[pfmooney]

Up until now, when a a Propolis instance is migrated, the rough "shape" of that instance (devices, etc) is created on the target VMM using the instance specification, after which it is "re-hydrated" using the resulting migration payload. Some of the details of that target instance depend on recreating identical data during initialization, such as fw_cfg contents (see #667) or the VPD info exposed via PCI devices.

It would be nice if the migration payloads were suitably enhanced to make it possible to reliably recreate an instance from only said migration payload (perhaps excluding basic details like vCPU count, etc).

Items which still require attention:

• fw_cfg data - fw_cfg device should migrate contents #667
• PCI VPD bits (vendor, model, etc)
• PCI capability layout