Some operating systems will generate a new device ID when a USB device is unplugged
and then replugged into the USB. If this is done whilst switching between multiple
applications over a virtual serial port, the change of device ID requires going
back into the OS/application to locate the new device accordingly.

Add a new always-plugged property that if specified will ensure that the device
always remains attached to the USB regardless of the state of the backend
chardev.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Message-id: 20201027150456.24606-7-mark.cave-ayland@ilande.co.uk
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>