virtio-devices: vhost_user: Adapt state() to return Result

This is a refactoring step in preparation for fetching backend device
state via SET_DEVICE_STATE_FD which can fail.

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

virtio-devices/src/vhost_user/net.rs

@@ -220,7 +220,7 @@ impl Net {
         })
     }
 
-    fn state(&self) -> State {
+    fn state(&self) -> std::result::Result<State, MigratableError> {
         self.vu_common.state(&self.common, self.config)
     }
 }
@@ -425,7 +425,7 @@ impl Snapshottable for Net {
     }
 
     fn snapshot(&mut self) -> std::result::Result<Snapshot, MigratableError> {
-        self.vu_common.snapshot(&self.state())
+        self.vu_common.snapshot(&self.state()?)
     }
 }
 impl Transportable for Net {}