usb/vmstate: add parent dev path

usb/vmstate: add parent dev path

... to make vmstate id string truely unique with multiple host
controllers, i.e. move from "1/usb-ptr" to "0000:00:01.3/1/usb-ptr"
(usb tabled connected to piix3 uhci).

This obviously breaks migration.  To handle this the usb bus
property "full-path" is added.  When setting this to false old
behavior is maintained.  This way current qemu will be compatible
with old versions when started using '-M pc-$oldversion'.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>