KVM: nVMX: do not pin the VMCS12

Since the current implementation of VMCS12 does a memcpy in and out
of guest memory, we do not need current_vmcs12 and current_vmcs12_page
anymore.  current_vmptr is enough to read and write the VMCS12.

And David Matlack noted:

  This patch also fixes dirty tracking (memslot->dirty_bitmap) of the
  VMCS12 page by using kvm_write_guest. nested_release_page() only marks
  the struct page dirty.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
[Added David Matlack's note and nested_release_page_clean() fix.]
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>