vmm: support setting cpu affinity with host cpu indices >255

On hosts with >256 cpus, setting the cpu affinity to a host cpu index
>255 will return an error because type of `host_cpu` is `u8`.
This commit changes the type of `host_cpu` to `usize` to remove this
limitation.

Signed-off-by: Sean Banko <sbanko@crusoeenergy.com>

vmm/src/vm_config.rs

@@ -10,7 +10,7 @@ use virtio_devices::RateLimiterConfig;
 #[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize)]
 pub struct CpuAffinity {
     pub vcpu: u8,
-    pub host_cpus: Vec<u8>,
+    pub host_cpus: Vec<usize>,
 }
 
 #[derive(Clone, Debug, Default, PartialEq, Eq, Deserialize, Serialize)]