Migrate all vhost-user devices to EpollHelper so as to remove code that
is duplicated between multiple virtio devices.
Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>
Split the generic virtio code (queues and device type) from the
VirtioDevice trait, transport and device implementations.
This also simplifies the feature handling in vhost_user_backend as the
vm-virtio crate is no longer has any features.
Signed-off-by: Rob Bradford <robert.bradford@intel.com>
