virtio-devices: Report no config space for configless devices

The watchdog, rng, and rtc devices expose no device specific
configuration fields. Each now reports a config size of zero so the
transport omits the device configuration capability instead of
advertising an unbacked region that the device cannot service.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

virtio-devices/src/rng.rs

@@ -230,6 +230,10 @@ impl Rng {
 }
 
 impl VirtioDevice for Rng {
+    fn config_size(&self) -> Option<u64> {
+        Some(0)
+    }
+
     fn device_type(&self) -> u32 {
         self.common.device_type
     }

virtio-devices/src/rtc.rs

@@ -601,6 +601,10 @@ impl Rtc {
 }
 
 impl VirtioDevice for Rtc {
+    fn config_size(&self) -> Option<u64> {
+        Some(0)
+    }
+
     fn device_type(&self) -> u32 {
         self.common.device_type
     }

virtio-devices/src/watchdog.rs

@@ -310,6 +310,10 @@ fn timerfd_setup(timer: &File, secs: i64) -> Result<(), io::Error> {
 }
 
 impl VirtioDevice for Watchdog {
+    fn config_size(&self) -> Option<u64> {
+        Some(0)
+    }
+
     fn device_type(&self) -> u32 {
         self.common.device_type
     }