monitor: Reset HMP mon->rs in CHR_EVENT_OPEN

Commit cdaa86a5 ("Add G_IO_HUP handler for socket chardev") exposed a bug in
the way the HMP monitor handles its command buffer. When a client closes the
connection to the monitor, tcp_chr_read() will detect the G_IO_HUP condition
and call tcp_chr_disconnect() to close the server-side connection too. Due to
the fact that monitor reads 1 byte at a time (for each tcp_chr_read()), the
monitor readline state / buffers might contain junk (i.e. a half-finished
command). Thus, without calling readline_restart() on mon->rs in
CHR_EVENT_OPEN, future HMP commands will fail.
Signed-off-by: NStratos Psomadakis <psomas@grnet.gr>
Signed-off-by: NDimitris Aragiorgis <dimara@grnet.gr>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>

monitor.c

@@ -5249,6 +5249,7 @@ static void monitor_event(void *opaque, int event)
         monitor_printf(mon, "QEMU %s monitor - type 'help' for more "
                        "information\n", QEMU_VERSION);
         if (!mon->mux_out) {
+            readline_restart(mon->rs);
             readline_show_prompt(mon->rs);
         }
         mon->reset_seen = 1;