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,
Maybe I did not not explain this clearly. 
I mean there's a <page> element in this class, so you can implement it by adding another XMLElementDict accessor.