device-manager: Fall back to reading from tty if no xkb layout specified

device-manager: Fall back to reading from tty if no xkb layout specified

At the moment if a user doesn't specify a xkb layout in vconsole.conf we
just fall back to assuming a us qwerty layout.

This isn't really optimal, since it might not match the keyboard.
Furthermore, the tty does have a usable layout, so it makes more sense
to fall back to it.

This commit changes device manager to disable reading from /dev/input
if the user doesn't specify a layout to use in vconsole.conf.

Close https://gitlab.freedesktop.org/plymouth/plymouth/-/issues/199