mmu_set_spte is called for either PTE prefetching or page faults.  The
three boolean arguments write_fault, speculative and host_writable are
always respectively false/true/true for prefetching and coming from
a struct kvm_page_fault for page faults.

Let mmu_set_spte distinguish these two situation by accepting a
possibly NULL struct kvm_page_fault argument.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>