WMI: embed struct device directly into wmi_block

Instead of creating wmi_blocks and then register corresponding devices
on a separate pass do it all in one shot, since lifetime rules for both
objects are the same. This also takes care of leaking devices when
device_create fails for one of them.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Matthew Garrett <mjg@redhat.com>