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>
