virtio-devices: Size the device config capability per device

Add a config_size method to VirtioDevice and use it when building the
PCI device configuration capability. The transport advertises the size
reported by the device and omits the capability entirely when the size
is zero, because the virtio driver rejects a zero length capability.

The method defaults to None, so every device keeps its current
capability size.

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

virtio-devices/src/device.rs

@@ -108,6 +108,10 @@ pub trait VirtioDevice: Send {
         let _ = value;
     }
 
+    fn config_size(&self) -> Option<u64> {
+        None
+    }
+
     /// Reads this device configuration space at `offset`.
     fn read_config(&self, _offset: u64, _data: &mut [u8]) {
         warn!(