vmm: config: Fix generic vhost-user parsing

The generic vhost-user device took its virtio device type on the
command line via the `virtio_id` parameter, but the same value is
called `device_type` in the API and the resulting config struct. This
irregularity was due to churn during the review process, `device_type`
was the intended name.

Accept `device_type` on the command line and keep `virtio_id` as a
deprecated alias that logs a warning. The alias will then be removed in
a later release.

Fixes: #8545

Assisted-by: Claude:Opus-4.8
Signed-off-by: Rob Bradford <rbradford@meta.com>

vmm/src/lib.rs

@@ -3590,7 +3590,7 @@ mod unit_tests {
     fn test_vmm_vm_cold_add_generic_vhost_user() {
         let mut vmm = create_dummy_vmm();
         let generic_vhost_user_config =
-            GenericVhostUserConfig::parse("virtio_id=26,socket=/tmp/sock,queue_sizes=[1024]")
+            GenericVhostUserConfig::parse("device_type=26,socket=/tmp/sock,queue_sizes=[1024]")
                 .unwrap();
 
         assert!(matches!(