libvirt_xml: Add memoryBacking tag for using hugepage in vm_xml.

The tag tells the hypervisor that the guest using hugepages,
instead of the normal native page size.
virttest.staging.utils_memory: Add functions to get/set hugepage parameters

These functions can be used to obtain the parameters of hugepage, such as
total/free/reserved memory,and enable/disable the transparent hugepage feature.
virttest.utils_misc.py : Add hugetlbfs for is_mounted()

It shouldn't be treated by os.path.realpath() while the file-type is hugetlbfs.