vm-virtio: net: Prepare NetQueuePair for use in vhost-user-net

This requires exposing the struct members and also using Option<..>
types for the main epoll fd and the memory as they are initialised later
in vhost-user-net.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vm-virtio/src/lib.rs

@@ -190,4 +190,5 @@ pub enum Error {
     EventfdError(io::Error),
     SetShmRegionsNotSupported,
     EpollHander(String),
+    NoMemoryConfigured,
 }