devices: wire close up as rpc method

Normally `DeviceService.close` would be called when the connection
was closed on the other end. Expose that close method via RPC so
that clients can call it explicitly. This should be mostly useful
in testing.

osbuild/devices.py

@@ -80,5 +80,8 @@ class DeviceService(host.Service):
         if method == "open":
             r = self.open(args["dev"], args["tree"], args["options"])
             return r, None
+        if method == "close":
+            r = self.close()
+            return r, None
 
         raise host.ProtocolError("Unknown method")