qemu-char: Inherit ptys and improve output from -serial pty

Changes since V1:
  - Avoid crashing since qemu_opts_id() may return null on some
    systems according to Markus's suggestion.

When controlling a qemu instance from another program, it's
hard to know which serial port or monitor device is redirected
to which pty. With more than one device using "pty" a lot of
guesswork is involved.

$ ./x86_64-softmmu/qemu-system-x86_64 -serial pty -serial pty -monitor pty
char device redirected to /dev/pts/5
char device redirected to /dev/pts/6
char device redirected to /dev/pts/7

Although we can find out what everything else is connected to
by the "info chardev" with "-monitor stdio" in the command line,
It'd be very useful to be able to have qemu inherit pseudo-tty
file descriptors so they could just be specified on the command
line like:

$ ./x86_64-softmmu/qemu-system-x86_64 -serial pty -serial pty -monitor pty
char device compat_monitor0 redirected to /dev/pts/5
char device serial0 redirected to /dev/pts/6
char device serial1 redirected to /dev/pts/7

Referred link: https://bugs.launchpad.net/qemu/+bug/938552

Signed-off-by: Lei Li <lilei@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

qemu-char.c

@@ -980,6 +980,7 @@ static CharDriverState *qemu_chr_open_pty(QemuOpts *opts)
     CharDriverState *chr;
     PtyCharDriver *s;
     struct termios tty;
+    const char *label;
     int master_fd, slave_fd, len;
 #if defined(__OpenBSD__) || defined(__DragonFly__)
     char pty_name[PATH_MAX];
@@ -1005,7 +1006,12 @@ static CharDriverState *qemu_chr_open_pty(QemuOpts *opts)
     chr->filename = g_malloc(len);
     snprintf(chr->filename, len, "pty:%s", q_ptsname(master_fd));
     qemu_opt_set(opts, "path", q_ptsname(master_fd));
-    fprintf(stderr, "char device redirected to %s\n", q_ptsname(master_fd));
+
+    label = qemu_opts_id(opts);
+    fprintf(stderr, "char device%s%s redirected to %s\n",
+            label ? " " : "",
+            label ?: "",
+            q_ptsname(master_fd));
 
     s = g_malloc0(sizeof(PtyCharDriver));
     chr->opaque = s;