x86/pv: Avoid locked bit manipulation in register_guest_callback()

x86/pv: Avoid locked bit manipulation in register_guest_callback()

Changes to arch.vgc_flags are made to current in syncrhonous context only, and
don't need to be locked.  (The only other changes are via
arch_set_info_guest(), which operates on descheduled vcpus only).

Replace the {set,clear}_bit() calls with compiler-visible bitwise operations.

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