udev: make sure auto-root logic also works in UKIs booted from XBOOTLDR

udev: make sure auto-root logic also works in UKIs booted from XBOOTLDR

If no root= switch is specified on the kernel command line we'll use the
root disk on which the partition the LoaderDevicePartUUID efi var is
located – as long as that partition is an ESP. Let's slightly liberalize
that and also allow it if that partition is an XBOOTLDR partition. This
ensures that UKIs spawned directly from XBOOTLDR work the same as those
from the ESP.

(Note that this makes no difference if sd-boot is in the mix, as in that
case LoaderDevicePartUUID is always set to the ESP, as that's where
sd-boot is located, and sd-boot will set the var first, sd-stub will
only set it later if it#s not set yet.)