xen/tmem: Convert the file common/tmem_xen.c to use typesafe MFN

xen/tmem: Convert the file common/tmem_xen.c to use typesafe MFN

The file common/tmem_xen.c is now converted to use typesafe. This is
requiring to override the macro page_to_mfn to make it work with mfn_t.

Note that all variables converted to mfn_t havem there initial value,
when set, switch from 0 to INVALID_MFN. This is fine because the initial
values was always overriden before used.

Also add a couple of missing newlines suggested by Andrew in the code.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>