pci: rename as_any to as_any_mut

That trait function returns a mutable reference. Rename it to follow
Rust's convention.

Signed-off-by: Wei Liu <liuwe@microsoft.com>

virtio-devices/src/transport/pci_device.rs

@@ -1249,7 +1249,7 @@ impl PciDevice for VirtioPciDevice {
         None
     }
 
-    fn as_any(&mut self) -> &mut dyn Any {
+    fn as_any_mut(&mut self) -> &mut dyn Any {
         self
     }