devices/loopback: better log message

It is more interesting which loopback device was used by which
host service instead of the file descriptor. Log that instead.

devices/org.osbuild.loopback

@@ -88,13 +88,14 @@ 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, lock)
         except Exception as error:  # pylint: disable: broad-except
             self.close()
             raise error from None
 
+        print(f"{self.lo.devname} acquired (locked: {lock})")
+
         dir_fd = -1
         try:
             dir_fd = os.open(devpath, os.O_CLOEXEC | os.O_PATH)