test/nm-service: default the devices to DISCONNETED not UNAVAILABLE

It is a little odd that client tests connect "UNAVAILABLE" devices, and
the devices return to "DISCONNECTED" after deactivation.

It differs from what happens in reality, and some client tools
(hey nm-cloud-setup) can break when they rightly assume that the
device is not ready for activation when it's "UNAVAILABLE" not
"DISCONNECTED".

tools/test-networkmanager-service.py

@@ -868,7 +868,7 @@ class Device(ExportedObj):
         self.activation_state_change_delay_ms = 50
         self.hwaddr = hwaddr is None if "" else hwaddr
 
-        self.prp_state = NM.DeviceState.UNAVAILABLE
+        self.prp_state = NM.DeviceState.DISCONNECTED
 
         if devtype == NM.DeviceType.MODEM:
             udi = "/org/freedesktop/ModemManager1/Modem/0"