virtio: mem: Implement device reset

The virtio-mem device does not have any backend specific state that
needs resetting so implement the method by simply returning true.
Plugged memory regions remain intact.

Signed-off-by: Ilias Stamatis <ilstam@amazon.com>

Author: ilstam

src/vmm/src/devices/virtio/mem/device.rs

@@ -660,6 +660,10 @@ impl VirtioDevice for VirtioMem {
         self.device_state = DeviceState::Inactive;
     }
 
+    fn _reset(&mut self) -> bool {
+        true
+    }
+
     fn activate(
         &mut self,
         mem: GuestMemoryMmap,