libxl: refactor DEFINE_DEVICE_REMOVE to fit for more device types

libxl: refactor DEFINE_DEVICE_REMOVE to fit for more device types

For some device type, device removal operation needs to be
handled specially, like usbctrl, it needs to remove all usb
devices under it first, then remove usbctrl. Extend
DEFINE_DEVICE_REMOVE to support generic and custom way
For those need to be handled specially, call
DEFINE_DEVICE_REMOVE_CUSTOM, it requires user defined
libxl__initiate_device_##type##_remove. Otherwise, just
call DEFINE_DEVICE_REMOVE as before.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Signed-off-by: Chunyan Liu <cyliu@suse.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>