fix a bug with hw devices.

if a device is unplugged and then replugged before we notice (via scan_devices) then it will get into an unusable state, throwing all kinds of low level exceptions when we don't expect it. affects ledger, keepkey, dbb, but for some reason not trezor.

lib/plugins.py

@@ -552,7 +552,7 @@ class DeviceMgr(ThreadJob, PrintError):
         with self.lock:
             connected = {}
             for client, pair in self.clients.items():
-                if pair in pairs:
+                if pair in pairs and client.has_usable_connection_with_device():
                     connected[client] = pair
                 else:
                     disconnected_ids.append(pair[1])