KVM: MMU: change kvm_faultin_pfn() arguments to kvm_page_fault

Add fields to struct kvm_page_fault corresponding to outputs of
kvm_faultin_pfn().  For now they have to be extracted again from struct
kvm_page_fault in the subsequent steps, but this is temporary until
other functions in the chain are switched over as well.

Suggested-by: Isaku Yamahata <isaku.yamahata@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>