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 <sf@sfritsch.de>
Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20170611074817.13621-1-sf@sfritsch.de>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  Use 'send a break' in all 3 pieces of text as suggested by eblake