vm-virtio: Improve virtio-net rx queue processing

The frame buffer must be updated depending on the amount read from it,
which depends on the number and depth of descriptors available at the
time of the processing.

This patch handles this buffer update, and allow for large buffers to be
correctly processed in multiple rounds.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vm-virtio/src/net_util.rs

@@ -430,7 +430,15 @@ impl RxVirtio {
         // Mark that we have at least one pending packet and we need to interrupt the guest.
         self.deferred_irqs = true;
 
-        write_count >= self.bytes_read
+        // Update the frame_buf buffer.
+        if write_count < self.bytes_read {
+            self.frame_buf.copy_within(write_count..self.bytes_read, 0);
+            self.bytes_read -= write_count;
+            false
+        } else {
+            self.bytes_read = 0;
+            true
+        }
     }
 }