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 
Yes, your commit is acceptable. But you can also refer to```class VMCPUTuneXML``` or ```class VMFeaturesXML```, which has the same situation with yours and no separate nest class defined.
And, given the more readable class name, I prefer 'VMMemBackingXML' to 'VMMBXML'.
Thanks.