X86/nVMX: Update the PML table without mapping and unmapping the page

Update the PML table without mapping and unmapping the page. This also
avoids using kvm_vcpu_gpa_to_page(..) which assumes that there is a "struct
page" for guest memory.

As a side-effect of using kvm_write_guest_page the page is also properly
marked as dirty.

Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>