Add a header with data structures and constants used in Hyper-V VMBus
hypervisor <-> guest interactions.

Based on the respective stuff from Linux kernel.

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: <20200424123444.3481728-3-arilou@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>