net_util, virtio-devices, vhost_user_net: Relocate code for opening TAP

By moving the code for opening the TAP device into a shared location we
are starting to remove the requirement for the vhost-user-net backend to
depend on the virtio-devices crate which in of itself depends on many
other crates that are not necessary for the backend to function.

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

net_util/Cargo.toml

@@ -5,12 +5,12 @@ authors = ["The Chromium OS Authors"]
 
 [dependencies]
 libc = "0.2.72"
+net_gen = { path = "../net_gen" }
 rand = "0.7.3"
 serde = "1.0.114"
+virtio-bindings = "0.1.0"
 vmm-sys-util = ">=0.3.1"
 
-net_gen = { path = "../net_gen" }
-
 [dev-dependencies]
 lazy_static = "1.3.0"
 pnet = "0.26.0"