KVM: Move x86's API to release a faultin page to common KVM

Move KVM x86's helper that "finishes" the faultin process to common KVM
so that the logic can be shared across all architectures.  Note, not all
architectures implement a fast page fault path, but the gist of the
comment applies to all architectures.

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: <20241010182427.1434605-50-seanjc@google.com>