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>

vmm/src/vm_config.rs

@@ -2,7 +2,7 @@
 //
 // SPDX-License-Identifier: Apache-2.0
 //
-use std::net::{IpAddr, Ipv4Addr};
+use std::net::IpAddr;
 use std::path::{Path, PathBuf};
 #[cfg(feature = "fw_cfg")]
 use std::str::FromStr;
@@ -303,10 +303,8 @@ pub fn default_diskconfig_queue_size() -> u16 {
 pub struct NetConfig {
     #[serde(default = "default_netconfig_tap")]
     pub tap: Option<String>,
-    #[serde(default = "default_netconfig_ip")]
-    pub ip: IpAddr,
-    #[serde(default = "default_netconfig_mask")]
-    pub mask: IpAddr,
+    pub ip: Option<IpAddr>,
+    pub mask: Option<IpAddr>,
     #[serde(default = "default_netconfig_mac")]
     pub mac: MacAddr,
     #[serde(default)]
@@ -352,20 +350,6 @@ pub fn default_netconfig_tap() -> Option<String> {
     None
 }
 
-pub fn default_netconfig_ip() -> IpAddr {
-    warn!(
-        "Deprecation warning: No IP address provided. A default IP address is assigned. This behavior will be deprecated soon."
-    );
-    IpAddr::V4(Ipv4Addr::new(192, 168, 249, 1))
-}
-
-pub fn default_netconfig_mask() -> IpAddr {
-    warn!(
-        "Deprecation warning: No network mask provided. A default network mask is assigned. This behavior will be deprecated soon."
-    );
-    IpAddr::V4(Ipv4Addr::new(255, 255, 255, 0))
-}
-
 pub fn default_netconfig_mac() -> MacAddr {
     MacAddr::local_random()
 }