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, will-Do
I really appreciate your patience and support.
I have tried to add <page ...> by XMLElementDict accessor, but it failed.
The reason is below,
There are two elements of <hugepages>, and they are all called <page>.
so if i try to add the second <page> to <hugepages> by XMLElementDict accessor, the first <tag> will be covered.

At last, I add the two <page>s by other way. Please take a look at the latest commit.
I wish you could accept it.
Thanks.