TRITON-2435 Preserve internal_metadata_namespaces property when migrating VMs.

[marsell]

If a VM is using internal_metadata_namespaces, and is then migrated, the internal_metadata_namespaces field is lost. Hilarity ensues.

Example VM before transfer:

[root@e4-43-4b-86-72-c8 (ro-1) ~]# vmadm get 486b163c-81a1-4dba-b22e-5315f3943242 | json internal_metadata internal_metadata_namespaces
{
  "foo:bar": "baz"
}
[
  "foo"
]

Example VM metadata in VMAPI before transfer:

[root@headnode (ro-1) ~]# sdc-vmapi /vms/486b163c-81a1-4dba-b22e-5315f3943242 | json -H internal_metadata internal_metadata_namespaces
{
  "foo:bar": "baz"
}
[
  "foo"
]

Performing the transfer:

[root@headnode (ro-1) ~]# sdc-migrate migrate -n 44454c4c-4400-105a-8037-c3c04f513033 486b163c-81a1-4dba-b22e-5315f3943242
# Migration begin running in job 6e583311-07bf-4da9-a1cc-60f624c4e8a3
 - reserving instance
 - syncing data
 - syncing data
  - running: 100%  1.1kB/s  (ETA 4s)
 - stopping the instance
 - syncing data
 - switching instances
 - filesytem sync finished, switching instances
 - reserving the IP addresses for the instance
 - setting up the target filesystem
 - hiding the original instance
 - promoting the migrated instance
 - removing sync snapshots
OK - switch was successful

Example VM after transfer:

[root@24-6e-96-2e-fa-54 (ro-1) ~]# vmadm get 486b163c-81a1-4dba-b22e-5315f3943242 | json internal_metadata internal_metadata_namespaces
{
  "foo:bar": "baz"
}

Example VM metadata in VMAPI after transfer:

[root@headnode (ro-1) ~]# sdc-vmapi /vms/486b163c-81a1-4dba-b22e-5315f3943242 | json -H internal_metadata internal_metadata_namespaces
{
  "foo:bar": "baz"
}

---

After this patch, the sdc-migration (in this case back to e4-43-4b-86-72-c8) preserves internal_metadata_namespaces:

[root@e4-43-4b-86-72-c8 (ro-1) ~]# vmadm get 486b163c-81a1-4dba-b22e-5315f3943242 | json internal_metadata internal_metadata_namespaces
{
  "foo:bar": "baz"
}
[
  "foo"
]

[root@headnode (ro-1) ~]# sdc-vmapi /vms/486b163c-81a1-4dba-b22e-5315f3943242 | json -H internal_metadata internal_metadata_namespaces
{
  "foo:bar": "baz"
}
[
  "foo"
]

[bahamat]

Filed internally: https://smartos.org/bugview/TRITON-2435

[bahamat]

This is currently blocked by #94, and will need to be rebased after that's merged.