context_tracking: Consolidate guest enter/exit wrappers

Consolidate the guest enter/exit wrappers, providing and tweaking stubs
as needed.  This will allow moving the wrappers under KVM without having
to bleed #ifdefs into the soon-to-be KVM code.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210505002735.1684165-7-seanjc@google.com