vmm: allow net devices without ip and mask

This change enables easier integration with third-party
tools by removing the requirement for a dummy IP address
when configuring tap devices. The modification applies to
both CLI and API interactions.

Previously, cloud-hypervisor would automatically set a
default static IP address (192.168.249.1) if none was provided.

This could lead to:

* multiple devices without explicit IP configurations
  would end up with the same default IP
* unnecessary inclusion of this IP in firewall rules
* the IP address could clash with host networking and
  routing

This introduces a new constraint:
When providing an IP, the mask must also be provided.

Removes warnings introduced in #7179.
Closes issue #7083.

Signed-off-by: Maximilian Güntner <code@mguentner.de>

src/main.rs

@@ -1336,20 +1336,6 @@ mod unit_tests {
                 }"#,
                 true,
             ),
-            (
-                vec![
-                    "cloud-hypervisor", "--kernel", "/path/to/kernel",
-                    "--net",
-                    "mac=12:34:56:78:90:ab,host_mac=34:56:78:90:ab:cd,tap=tap0,ip=1.2.3.4",
-                ],
-                r#"{
-                    "payload": {"kernel": "/path/to/kernel"},
-                    "net": [
-                        {"mac": "12:34:56:78:90:ab", "host_mac": "34:56:78:90:ab:cd", "tap": "tap0", "ip": "1.2.3.4"}
-                    ]
-                }"#,
-                true,
-            ),
             (
                 vec![
                     "cloud-hypervisor", "--kernel", "/path/to/kernel",