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 .
@PandaWei 
Thanks for your work. But about class 'VMMemBackingXML', it still need some updates:
1. Number of element '<page/>' >=0, which can deal with like 'vcpupins' in class VMCPUTuneXML(use  accessors.XMLElementList) or 'feature_list' in class VMFeaturesXML, but <page> is under <hugepages>, not the top level, so the members in this class served for 'page' are need update, you can just add those you need()such as 'get_page', 'add_page', etc.), and let other to patch it when need new funcs.
2. For  <nosharepages/> and <locked/>, your original code is good enough, just implement them by a XMLElementBool accessor.