KVM: VMX: always require WB memory type for EPT

We already always set that type but don't check if it is supported. Also
for nVMX, we only support WB for now. Let's just require it.

Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>