virttest.staging.utils_memory: strengthen get_num_anon_huge_pages()

This function is intended to obtain the AnonHugePages usage of VM.
Sometime, we want to get the AnonHugePages usage of Host,
so this patch can achieve this purpose.
Merge pull request #1903 from PandaWei/anonhugepage

virttest.staging.utils_memory: strengthen get_num_anon_huge_pages()