KVM: nVMX: get rid of nested_release_page*

Let's also just use the underlying functions directly here.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
[Rebased on top of 9f744c59 ("KVM: nVMX: do not pin the VMCS12")]
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>