virtio-devices: Add feature acked gated accessor for AccessPlatform

Add VirtioCommon::access_platform() method. The virtio spec requires
that only if the feature is acked should the accesses be transformed via
the access platform implementation. This will enable that filtering.

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

virtio-devices/src/device.rs

@@ -16,6 +16,7 @@ use std::thread;
 use anyhow::anyhow;
 use libc::EFD_NONBLOCK;
 use log::{error, info, warn};
+use virtio_bindings::virtio_config::VIRTIO_F_ACCESS_PLATFORM;
 use virtio_queue::Queue;
 use vm_device::UserspaceMapping;
 use vm_memory::{GuestAddress, GuestMemoryAtomic};
@@ -332,6 +333,15 @@ impl VirtioCommon {
         // requires the addresses held by the descriptors to be translated.
         self.avail_features &= !(1 << VIRTIO_F_RING_INDIRECT_DESC);
     }
+
+    /// Returns the access platform only if the feature has been acked.
+    pub fn access_platform(&self) -> Option<Arc<dyn AccessPlatform>> {
+        if self.feature_acked(VIRTIO_F_ACCESS_PLATFORM as u64) {
+            self.access_platform.clone()
+        } else {
+            None
+        }
+    }
 }
 
 impl Pausable for VirtioCommon {