vsock: get rid of compilation warnings
vsock: get rid of compilation warnings

warning: unused import: `vhost_gen::vhost::*`
  --> devices/src/virtio/vhost/vsock.rs:16:5
   |
16 | use vhost_gen::vhost::*;
   |     ^^^^^^^^^^^^^^^^^^^
   |
   = note: #[warn(unused_imports)] on by default

warning: unused import: `virtio_gen::virtio_config::*`
  --> devices/src/virtio/vhost/vsock.rs:17:5
   |
17 | use virtio_gen::virtio_config::*;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: unused import: `virtio_gen::virtio_ring::*`
  --> devices/src/virtio/vhost/vsock.rs:18:5
   |
18 | use virtio_gen::virtio_ring::*;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^

Signed-off-by: Liu Jiang <aaf6961e30ae80fd3a577c4cb6742ab6a1ad13eb@linux.alibaba.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker