add new -mon switch

add new -mon switch

Add -mon switch which maps pretty straight forward into the QemuOpts
internal representation:

  -mon chardev=<name>[,mode=[control|readline]][,[no]default]

Via config file:

[mon]
   chardev = "<name>"
   mode = "readline"
   default = "on"

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
(cherry picked from commit 22a0e04b9bb5a02e13b3e5cf5ea8abfac5f34120)