virtio-devices: Add VirtioDevice::access_platform()

Adding this method to the trait will allow the virtio PCI code to access
a feature conditional version of the access platform and simplify the
logic.

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

virtio-devices/src/device.rs

@@ -188,6 +188,12 @@ pub trait VirtioDevice: Send {
     /// Set the access platform trait to let the device perform address
     /// translations if needed.
     fn set_access_platform(&mut self, _access_platform: Arc<dyn AccessPlatform>) {}
+
+    /// Returns the access platform only if VIRTIO_F_ACCESS_PLATFORM was
+    /// negotiated with the guest.
+    fn access_platform(&self) -> Option<Arc<dyn AccessPlatform>> {
+        None
+    }
 }
 
 /// Trait to define address translation for devices managed by virtio-iommu