intel_iommu: allow dev-iotlb context entry conditionally

When device-iotlb is not specified, we should fail this check. A new
function vtd_ce_type_check() is introduced.

While I'm at it, clean up the vtd_dev_to_context_entry() a bit - replace
many "else if" usage into direct if check. That'll make the logic more
clear.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
