VMCI: doorbell implementation.

VMCI doorbell code allows for notifcations between host and guest.

Signed-off-by: George Zhang <[email protected]>
Acked-by: Andy king <[email protected]>
Acked-by: Dmitry Torokhov <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: georgezhang11