xen/tmem: s/disable_// and change the logic.

The variety of disable_[cleancache|frontswap|selfshrinking] are
making this a bit complex. Just remove the "disable_" part and
change the logic around for the "nofrontswap" and "nocleancache"
parameters.

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