rework -monitor handling, switch to QemuOpts

This patch reworks the -monitor handling:

 - It adds a new "mon" QemuOpts list for the monitor(s).
 - It adds a monitor_parse() function to parse the -monitor switch.
 - It adds a mon_init function to initialize the monitor(s) from the
   "mon" QemuOpts list.
 - It winds up everything and removes the old bits.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
