vmm: config: Switch NetConfig to use PciDeviceCommonConfig

Switch NetConfig over to using the newly extracted struct members as
used by all PCI based devices. The use of #[serde(flatten)] means that
this change has no impact on the JSON format that the data is stored as.

Signed-off-by: Rob Bradford <rbradford@meta.com>

vmm/src/api/http/http_endpoint.rs

@@ -122,7 +122,7 @@ mod fds_helper {
 
         impl ConfigWithFDs for NetConfig {
             fn id(&self) -> Option<&str> {
-                self.id.as_deref()
+                self.pci_common.id.as_deref()
             }
 
             fn fds_from_http_body(&self) -> Option<&[RawFd]> {