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 .
@will-Do 
you mean following？
<hugepages>    :  add  it by XMLElementBool()?
      <page size="1" unit="G" nodeset="0-3,5"/>    add it by add_page?
      <page size="2" unit="M" nodeset="4"/>          
</hugepages>

is it right?