server: get migration target specs from their sources #807
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Broken out from #806; related to #804.
Instead of checking the compatibility of migration source and target instance specs, have targets initialize from amended versions of their sources' specs. The benefits of doing this are that
Some components of the source spec, notably its Crucible and viona backends, do need to change during migration: Crucible backends need new generation numbers, and viona backends need the VNIC names that the target sled assigned to them. To accommodate this, the sync phase substitutes the target's Crucible and viona backend descriptors into the source's spec. The forthcoming API changes in the rest of #806 will formalize this more, but the current approach is sufficient to keep the PHD Crucible migration tests passing.
In the future, migration targets won't have a spec until the migration protocol has started. Prepare for this change as follows:
instance_spec_get
API to reflect that a spec might not be available if a VM is initializing via live migration.Tests: cargo test, PHD.