The mailbox handler drained the FIFO without checking the message body.
That caused shutdown messages to be lost, hence being unable to start/stop
the R5 cores from the Linux side.
The additions allow checking messages for control and virtqueue types.
Shutdown messages fall to the control branch, which ACKs the shutdown
request and parks the core in WFI.
Virtqueues still work as intended; the only difference is that control
messages are now handled correctly.
Assisted-by: Claude Code:claude-opus-5
Signed-off-by: Ulaş Sertan Kemeç <sertan.usk@gmail.com>
2cb30ac13d