scripts/runqemu: allocate unfsd ports in a way that doesn't race or clash with unrelated processes

There is already a neat check_free_port() function for finding an available port
atomically, so use that and make two additional tweaks:

- no need to allocate two separate ports; per unfsd documentation they can be the same

- move lockfile release until after unfsd has been shut down and the port(s) used has been freed

[YOCTO #15077]

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>