vm-virtio, virtio-devices: Split device implementation from virt queues

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>

vmm/src/config.rs

@@ -3,8 +3,6 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 
-extern crate vm_virtio;
-
 use clap::ArgMatches;
 use net_util::MacAddr;
 use std::collections::HashMap;