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>

option_parser/src/lib.rs

@@ -36,7 +36,7 @@ struct OptionParserValue {
     requires_value: bool,
 }
 
-#[derive(Error, Debug)]
+#[derive(Debug, Error)]
 pub enum OptionParserError {
     #[error("unknown option: {0}")]
     UnknownOption(String),