KVM: x86/mmu: Convert page fault paths to kvm_faultin_pfn()

Convert KVM x86 to use the recently introduced __kvm_faultin_pfn().
Opportunstically capture the refcounted_page grabbed by KVM for use in
future changes.

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: <20241010182427.1434605-45-seanjc@google.com>