vmm: Introduce option --platform iommufd=on|off

This option allows user to configure VFIO device pass-through with
iommufd (e.g. vfio cdev mode) or not (e.g. vfio legacy mode).

Signed-off-by: Bo Chen <bchen@crusoe.ai>

vmm/src/vm_config.rs

@@ -133,6 +133,8 @@ pub struct PlatformConfig {
     #[cfg(feature = "sev_snp")]
     #[serde(default)]
     pub sev_snp: bool,
+    #[serde(default)]
+    pub iommufd: bool,
 }
 
 pub const DEFAULT_PCI_SEGMENT_APERTURE_WEIGHT: u32 = 1;