|||0||RW||msg_rcv||For VF: VF marks the message in its Incoming Mailbox Memory as
received. Hardware asserts the acknowledgement bit of the associated PF.
For PF: PF marks the message send by target_fn as received. The hardware will refresh the i_msg_status of the PF, and clear the o_msg_status of the target_fn.
|||0||RW||msg_send||For VF: VF marks the current message in its own Outgoing
Mailbox as valid.