net_util: Support supplying flags to open_tap() helper

This helper can open a TAP device and configure the interface on it. If
the device needs to be opened multiple times for MQ then it also handles
that correctly.

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

vhost_user_net/src/lib.rs

@@ -143,6 +143,7 @@ impl VhostUserNetBackend {
             Some(netmask),
             &mut Some(host_mac),
             num_queues / 2,
+            None,
         )
         .map_err(Error::OpenTap)?;