vmm: Introduce a PciDeviceCommonConfig struct

Introduce a common struct that can encompass all the config fields
used by devices that are PCI based. The use of `skip_serializing_if`
means that the iommu field will only be included if set (otherwise
falling back to default false). This neatly handles the devices that
don't support an iommu.

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

vmm/src/vm_config.rs

@@ -273,6 +273,16 @@ pub struct VirtQueueAffinity {
     pub host_cpus: Vec<usize>,
 }
 
+#[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize, Default)]
+pub struct PciDeviceCommonConfig {
+    #[serde(default)]
+    pub id: Option<String>,
+    #[serde(default, skip_serializing_if = "<&bool as std::ops::Not>::not")]
+    pub iommu: bool,
+    #[serde(default)]
+    pub pci_segment: u16,
+}
+
 #[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize)]
 pub struct DiskConfig {
     pub path: Option<PathBuf>,