vmm: config: Extend 'VmConfig' with 'preserved_fds'

Preserved FDs are the ones that share the same life-time as its holding
VmConfig instance, such as FDs for creating TAP devices.

Preserved FDs will stay open as long as the holding VmConfig instance is
valid, and will be closed when the holding VmConfig instance is destroyed.

Signed-off-by: Bo Chen <chen.bo@intel.com>

src/main.rs

@@ -722,6 +722,7 @@ mod unit_tests {
             gdb: false,
             platform: None,
             tpm: None,
+            preserved_fds: None,
         };
 
         assert_eq!(expected_vm_config, result_vm_config);