core, restapi: cloud-init [2/6] - custom volume id in VM Payload

Change required for cloud-init integration: allow disks created for
VM payloads to have custom volume labels.

Change-Id: I30a3be934869c62ed4cdd9942e7fe17af67572e0
Signed-off-by: Greg Padgett <gpadgett@redhat.com>