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 .
It's just an example in libvirt.org, so no need hard code these 2 pages here.
BTW, 'hugepage' can take no page, so I suggest you can set 'hugepage' just like 'nosharepages' and 'locked', so after call this function, domain XML has following elements:
```
  <memoryBacking>
    </hugepages>
    <nosharepages/>
    <locked/>
  </memoryBacking>
```