KVM: x86/mmu: Add helper to "finish" handling a guest page fault

Add a helper to finish/complete the handling of a guest page, e.g. to
mark the pages accessed and put any held references.  In the near
future, this will allow improving the logic without having to copy+paste
changes into all page fault paths.  And in the less near future, will
allow sharing the "finish" API across all architectures.

No functional change intended.

Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20241010182427.1434605-41-seanjc@google.com>