Support for multiple -monitor devices

Support for multiple -monitor devices

Rebased version of Anthony's patch: Allow to specify more than one
monitor terminal via the -monitor command line switch. This is
particularly useful when libvirt or some other management tool already
occupies the primary monitor but you need another one for debugging.
The current clumsy workaround is to multiplex such additional terminals
over a qemu character device (e.g. -serial mon:<device>).

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>