xen/tmem: cleanup

Use 'bool' for boolean variables. Do proper section placement.
Eliminate an unnecessary export.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>