x86/pv: Move x86/trace.c to x86/pv/trace.c

x86/pv: Move x86/trace.c to x86/pv/trace.c

This entire file is pv-only, and not excluded from the build by
CONFIG_TRACEBUFFER.  Move it into the pv/ directory, build it conditionally,
and drop unused includes.

Also move the contents of asm/trace.h to asm/pv/trace.h to avoid the functions
being declared across the entire hypervisor.

One caller in fixup_page_fault() is effectively PV only, but is not subject to
dead code elimination.  Add an additional IS_ENABLED(CONFIG_PV) to keep the
build happy.

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