I introduced a regression: when users didn't specify a specific name via
lxc.net.<idx>.name then the device would retain the random name it received
when we created it. Before we would use the "eth%d" syntax to get the kernel to
assign a fixed name. Restore that behavior.

Closes: #3407.
Fixes: 8bf64b77 ("network: rework network device creation")
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>