Some virtio devices cannot be implemented via vhost-user because they
require tight integration with the VMM. This includes the IOMMU and
watchdog devices.
An attempt to create a generic vhost-user device with one of these IDs
is always either a bug or human error. To aid debugging, return a
helpful error message rather than silently continuing.
Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>