block: fix status value size

As per VirtIO spec 1.2 section 5.2.6, the `status` field is a byte, not
u32. cloud-hypervisor writes an `u32` to guest memory, which
accidentally zeros out the following 3 bytes, and may corrupt guest OS
internal state.

Signed-off-by: Changyuan Lyu <changyuanl@google.com>

vhost_user_block/src/lib.rs

@@ -148,7 +148,7 @@ impl VhostUserBlkThread {
                     ) {
                         Ok(l) => {
                             len = l;
-                            VIRTIO_BLK_S_OK
+                            VIRTIO_BLK_S_OK as u8
                         }
                         Err(e) => {
                             len = 1;