devices/loopback: add some info message

Show the file descriptor that was opened for the file passed to
the device. Recently, in CI, we have seen errors opening the
loop device with `fd` being `-1` and this ensures that at least
the file itself could be opened.

devices/org.osbuild.loopback

@@ -99,6 +99,7 @@ class LoopbackService(devices.DeviceService):
         path = os.path.join(tree, filename.lstrip("/"))
 
         self.fd = os.open(path, os.O_RDWR | os.O_CLOEXEC)
+        print(f"file '{filename}'' opened as {self.fd}")
         try:
             self.lo = self.make_loop(self.fd, start, size)
         except Exception as error:  # pylint: disable: broad-except