xsm: Use the dummy XSM module if XSM is disabled

xsm: Use the dummy XSM module if XSM is disabled

This patch moves the implementation of the dummy XSM module to a
header file that provides inline functions when XSM_ENABLE is not
defined. This reduces duplication between the dummy module and callers
when the implementation of the dummy return is not just "return 0",
and also provides better compile-time checking for completeness of the
XSM implementations in the dummy module.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Committed-by: Keir Fraser <keir@xen.org>