xen-balloon: convert sysdev_class to a regular subsystem

After all sysdev classes are ported to regular driver core entities, the
sysdev implementation will be entirely removed from the kernel.

Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>