virttest: Remove the special strings from sysfs of numa possible node

In some system the possible node information read from sysfs include some
special string as \x00 for end of the string. Need remove it as it will
failed the translate of string to int.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #1677 from ypu/numa_fix

virttest: Remove the special strings from sysfs of numa possible node