devices: Enable build with kvm feature

Currently `devices` module cannot be solely built, by specifying
`arch/kvm` to turn on the features required for its dependency - `arch`
crate to build. Thus enabling `devices` crate to be built and tested
with command:

```sh
cargo build -p devices --features kvm
```

Signed-off-by: Ruoqing He <heruoqing@iscas.ac.cn>

devices/Cargo.toml

@@ -34,4 +34,5 @@ arch = { path = "../arch" }
 
 [features]
 default = []
+kvm = ["arch/kvm"]
 pvmemcontrol = []