WMI: Cater for multiple events with same GUID

WMI data blocks can contain WMI events with the same GUID but with
different notifiy_ids, for example volume up/down hotkeys.
This patch enables a single event handler to be registered and
unregistered against all events with same GUID but different
notify_ids.  Since an event handler is passed the notify_id of
an event it can can differentiate between the different events.

The patch also ensures we only register and unregister a device per
unique GUID.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>