libvirt.tests: Fix numatune test bugs.

Checking numa may fail because len(cpus_parse...) will return
wrong expected node number when type is "excluding".
"0-2,^1" for cpus_parse() will return 2 instead of 3 which is expected, if
there are 2 nodes on machine, it will cause unexpected failed.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>