vmm: Allow IP configuration on named TAP interfaces

This commit changes existing behavior of named TAP interfaces.
When booting a VM with configuration for a named TAP interface,
cloud-hypervisor will create the interface and apply a given
IP configuration to that interface. If the named interface
already exists on the system, the configuration is NOT overwritten.

Setting the ip and netmask fields in a tap interface configuration
for a named tap interface now works by handing this configuration
to the virtio_devices::Net object when it is created with a name.

This commit also touches net_util to make sure that the ip configuration
of existing TAP interfaces is not modified with ip or netmask handed to
open_tap.

Signed-off-by: Markus Sütter <markus.suetter@secunet.com>

vmm/src/device_manager.rs

@@ -2464,8 +2464,8 @@ impl DeviceManager {
                     virtio_devices::Net::new(
                         id.clone(),
                         Some(tap_if_name),
-                        None,
-                        None,
+                        Some(net_cfg.ip),
+                        Some(net_cfg.mask),
                         Some(net_cfg.mac),
                         &mut net_cfg.host_mac,
                         net_cfg.mtu,