xen: balloon: Use static attribute groups for sysfs entries

Instead of manual calls of device_create_file(), device_remove_file()
and sysfs_create_group(), assign static attribute groups to the device
to register.  This simplifies the code and avoids possible races.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>