vm-virtio: Simplify virtio feature handling

Remove duplicated code across the different devices by handling
the virtio feature pages in VirtioDevice itself rather than
in the backends. This works as no virtio devices use feature
bits beyond 64-bits.

Signed-off-by: Cathy Zhang <cathy.zhang@intel.com>

vm-virtio/src/device.rs

@@ -62,14 +62,15 @@ pub trait VirtioDevice: Send {
     /// The maximum size of each queue that this device supports.
     fn queue_max_sizes(&self) -> &[u16];
 
-    /// The set of feature bits shifted by `page * 32`.
-    fn features(&self, page: u32) -> u32 {
-        let _ = page;
+    /// The set of feature bits that this device supports.
+    fn features(&self) -> u64 {
         0
     }
 
     /// Acknowledges that this set of features should be enabled.
-    fn ack_features(&mut self, page: u32, value: u32);
+    fn ack_features(&mut self, value: u64) {
+        let _ = value;
+    }
 
     /// Reads this device configuration space at `offset`.
     fn read_config(&self, offset: u64, data: &mut [u8]);