xen/tmem: Split out the different module/boot options.

There are three options - depending on what combination of
CONFIG_FRONTSWAP, CONFIG_CLEANCACHE and CONFIG_XEN_SELFBALLOONING
is used. Lets split them out nicely out in three groups to
make it easier to clean up.

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