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 .
(Sorry, i didn't deal with markdown elements in last comment, so some words not show up correctly, just updated it.)
Yes,  a  XMLElementBool() accessor for ```<hugepages>``` is good, and that means there's no ```<page>```, just like  ```<nosharepages/>``` and ```<locked/>```.
And, there are two ways to implement the ```<page>``` list, one is like ```class VMCPUTuneXML```, by adding a XMLElementList:
```
accessors.XMLElementList('property_name='pages', 
                         libvirtxml=self,
                         parent_xpath='hugepages',
                         marshal_from=self.marshal_from_pages,
                         marshal_to=self.marshal_to_pages)
def marshal_from_pages():
...
def marshal_to_pages():
...
```
Another way is like ```class VMFeaturesXML```, use class function members to add/del ```<page/>```, which dependent on you demand:
```
accessors.AllForbidden(property_name="pages", libvirtxml=self)

def get_pages():
...
def add_page():
...
def del_page():
...
```