x86/shadow: Rework trace_shadow_gen() into sh_trace_va()

x86/shadow: Rework trace_shadow_gen() into sh_trace_va()

The ((GUEST_PAGING_LEVELS - 2) << 8) expression in the event field is common
to all shadow trace events, so introduce sh_trace() as a very thin wrapper
around trace().

Then, rename trace_shadow_gen() to sh_trace_va() to better describe what it is
doing, and to be more consistent with later cleanup.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-Acked-By: Oleksii Kurochko <oleksii.kurochko@gmail.com>