xen/tmem: Remove the usage of '[no|]selfballoon' and use 'tmem.selfballooning' bool instead.

As the 'tmem' driver is the one that actually sets whether
it will use it (or not) so might as well make tmem responsible
for this knob.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>