Add chardev-send-break monitor command

Add chardev-send-break monitor command

Sending a break on a serial console can be useful for debugging the
guest. But not all chardev backends support sending breaks (only telnet
and mux do). The chardev-send-break command allows to send a break even
if using other backends.

Signed-off-by: Stefan Fritsch <[email protected]>
Acked-by: Dr. David Alan Gilbert <[email protected]>
Message-Id: <20170611074817[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>
Signed-off-by: Dr. David Alan Gilbert <[email protected]>
  Use 'send a break' in all 3 pieces of text as suggested by eblake