feat(net): add MRG_RXBUF support to virtio-net device

Now virtio-net device VIRTIO_NET_F_MRG_RXBUF feature which allows it
to write single packet into multiple descriptor chains.
The amount of descriptor chains (also known as heads) is written into
the `virtio_net_hdr_v1` structure which is located at the very begging
of the packet.

Signed-off-by: Egor Lazarchuk <[email protected]>

src/vmm/src/devices/virtio/iovec.rs

@@ -367,10 +367,15 @@ impl IoVecBufferMut {
     /// In contrast to the equivalent [`IoVecBuffer::len()`] which returns `u32`, this one returns
     /// `usize` since the buffer can contain multiple `DescriptorChain` objects, so we don't have
     /// the limit that the length of a buffer is limited by `u32`.
-    pub(crate) fn len(&self) -> usize {
+    pub fn len(&self) -> usize {
         self.len
     }
 
+    /// Returns true if there is buffer is empty.
+    pub fn is_empty(&self) -> bool {
+        self.len == 0
+    }
+
     /// Returns a pointer to the memory keeping the `iovec` structs
     pub fn as_iovec_mut_slice(&mut self) -> &mut [iovec] {
         self.vecs.as_mut_slice()