vsock: Increase NUM_QUEUES to 3

In virtio standard, vsock uses 3 vqs. crosvm expects 3 vqs from
vhost-user-vsock impl, but this vhost-user-vsock device sets up
only 2 vqs because event vq isn't handled. And it causes crash in
crosvm. To avoid crash in crosvm, I increase NUM_QUEUES to 3

Signed-off-by: Jeongik Cha <jeongik@google.com>

crates/vsock/src/vhu_vsock.rs

@@ -26,7 +26,7 @@ use crate::vhu_vsock_thread::*;
 
 pub(crate) type CidMap = HashMap<u64, (Arc<RwLock<RawPktsQ>>, EventFd)>;
 
-const NUM_QUEUES: usize = 2;
+const NUM_QUEUES: usize = 3;
 const QUEUE_SIZE: usize = 256;
 
 // New descriptors pending on the rx queue