vmbus: add infrastructure to save/load vmbus requests

This can be allow to include controller-specific data while
saving/loading in-flight scsi requests of the vmbus scsi controller.

Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Signed-off-by: Jon Doron <arilou@gmail.com>
Message-Id: <20200424123444.3481728-7-arilou@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>