vm-virtio, virtio-devices: Address Rust 1.51.0 clippy issue (upper_case_acronyms)

error: name `TYPE_UNKNOWN` contains a capitalized acronym
  --> vm-virtio/src/lib.rs:48:5
   |
48 |     TYPE_UNKNOWN = 0xFF,
   |     ^^^^^^^^^^^^ help: consider making the acronym lowercase, except the initial letter: `Type_Unknown`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#upper_case_acronyms

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

virtio-devices/src/mem.rs

@@ -816,7 +816,7 @@ impl Mem {
 
         Ok(Mem {
             common: VirtioCommon {
-                device_type: VirtioDeviceType::TYPE_MEM as u32,
+                device_type: VirtioDeviceType::Mem as u32,
                 avail_features,
                 paused_sync: Some(Arc::new(Barrier::new(2))),
                 queue_sizes: QUEUE_SIZES.to_vec(),