qdev hotplug: infrastructure and monitor commands.

Adds device_add and device_del commands.  device_add accepts accepts
the same syntax like the -device command line switch.  device_del
expects a device id.  So you should tag your devices with ids if you
want to remove them later on, like this:

  device_add pci-ohci,id=ohci
  device_del ohci

Unplugging via pci_del or usb_del works too.

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