Add handling of POST_MESSAGE hypercall. For that, add an interface to
regsiter a handler for the messages arrived from the guest on a
particular connection id (IOW set up a message connection in Hyper-V
speak).
Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
Message-Id: <20180921082217.29481-10-rkagan@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>