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/vhost_user/blk.rs

@@ -139,7 +139,7 @@ impl Blk {
 
         Ok(Blk {
             common: VirtioCommon {
-                device_type: VirtioDeviceType::TYPE_BLOCK as u32,
+                device_type: VirtioDeviceType::Block as u32,
                 queue_sizes: vec![vu_cfg.queue_size; vu_cfg.num_queues],
                 avail_features,
                 acked_features,

virtio-devices/src/vhost_user/fs.rs

@@ -356,7 +356,7 @@ impl Fs {
 
         Ok(Fs {
             common: VirtioCommon {
-                device_type: VirtioDeviceType::TYPE_FS as u32,
+                device_type: VirtioDeviceType::Fs as u32,
                 avail_features,
                 acked_features,
                 queue_sizes: vec![queue_size; num_queues],

virtio-devices/src/vhost_user/net.rs

@@ -151,7 +151,7 @@ impl Net {
         Ok(Net {
             id,
             common: VirtioCommon {
-                device_type: VirtioDeviceType::TYPE_NET as u32,
+                device_type: VirtioDeviceType::Net as u32,
                 queue_sizes: vec![vu_cfg.queue_size; queue_num],
                 avail_features,
                 acked_features,