vm-virtio: vsock: add is_empty method to VsockPacket

This patch adds `is_empty` method to VsockPacket to fix the
following clippy error:

error: item `vsock::packet::VsockPacket` has a public `len` method but no corresponding `is_empty` method
   --> vm-virtio/src/vsock/packet.rs💯1
    |
100 | / impl VsockPacket {
101 | |     /// Create the packet wrapper from a TX virtq chain head.
102 | |     ///
103 | |     /// The chain head is expected to hold valid packet header data. A following packet buffer
...   |
334 | |     }
335 | | }
    | |_^
    |
    = note: `-D clippy::len-without-is-empty` implied by `-D warnings`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#len_without_is_empty

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>

vm-virtio/src/vsock/packet.rs

@@ -124,7 +124,7 @@ impl VsockPacket {
         };
 
         // No point looking for a data/buffer descriptor, if the packet is zero-lengthed.
-        if pkt.len() == 0 {
+        if pkt.is_empty() {
             return Ok(pkt);
         }
 
@@ -278,6 +278,10 @@ impl VsockPacket {
         LittleEndian::read_u32(&self.hdr()[HDROFF_LEN..])
     }
 
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
     pub fn set_len(&mut self, len: u32) -> &mut Self {
         LittleEndian::write_u32(&mut self.hdr_mut()[HDROFF_LEN..], len);
         self