Patching virttest to facilitate the test of hugepage feature

Aim to make the guest using hugepages, adding memoryBacking tag in libvirt_xml/vm_xml.
Further, adding functions used to get/set the parameter of hugepage in virttest.staging.utils_memory .
Hi, will-Do
Thank you for your advice in advance. it's nice.
But I want to keep unanimous with the style of the original text. such as the implement of 'pm', 'cputune',and so on.