run.py: fix issue with memory size parsing for xen

I think the current code is broken in 'GB' case because it trims
memory string twice.

Spotted during skimming over the file.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>