xen: Move async_exception_* infrastructure into x86

xen: Move async_exception_* infrastructure into x86

The async_exception_{state,mask} infrastructure is implemented in common code,
but is limited to x86 because of the VCPU_TRAP_LAST ifdef-ary.

The internals are very x86 specific (and even then, in need of correction),
and won't be of interest to other architectures.  Move it all into x86
specific code.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>