runtime-rs: Allow enable-vendor feature

Clippy fails with:
```
error: unexpected `cfg` condition value: `enable-vendor`
   --> crates/hypervisor/src/device/driver/vfio.rs:180:11
    |
180 |     #[cfg(feature = "enable-vendor")]
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: expected values for `feature` are: `ch-config`, `cloud-hypervisor`, `default`, and `dragonball`
    = help: consider adding `enable-vendor` as a feature in `Cargo.toml`
```

So add it as an expected cfg in the linter to skip this

Signed-off-by: stevenhorsman <steven@uk.ibm.com>

src/runtime-rs/crates/hypervisor/Cargo.toml

@@ -51,7 +51,20 @@ qapi-spec = "0.3.1"
 qapi-qmp = "0.14.0"
 
 [target.'cfg(not(target_arch = "s390x"))'.dependencies]
-dragonball = { path = "../../../dragonball", features = ["atomic-guest-memory", "virtio-vsock", "hotplug", "virtio-blk", "virtio-net", "virtio-fs", "vhost-net", "dbs-upcall", "virtio-mem", "virtio-balloon", "vhost-user-net", "host-device"], optional = true }
+dragonball = { path = "../../../dragonball", features = [
+    "atomic-guest-memory",
+    "virtio-vsock",
+    "hotplug",
+    "virtio-blk",
+    "virtio-net",
+    "virtio-fs",
+    "vhost-net",
+    "dbs-upcall",
+    "virtio-mem",
+    "virtio-balloon",
+    "vhost-user-net",
+    "host-device",
+], optional = true }
 dbs-utils = { path = "../../../dragonball/src/dbs_utils" }
 hyperlocal = "0.8.0"
 hyper = { version = "0.14.18", features = ["client"] }
@@ -76,3 +89,8 @@ serial_test = "2.0.0"
 
 [build-dependencies]
 ttrpc-codegen = "0.4.2"
+
+[lints.rust]
+unexpected_cfgs = { level = "warn", check-cfg = [
+    'cfg(feature, values("enable-vendor"))',
+] }