libvirt: enhance config memory backing to handle hugepages

Introduces a new config LibvirtConfigGuestMemoryBackingPage and
updates config LibvirtConfigGuestMemoryBacking to take advantage
of hugepages allocation for guests.

Work-Item: Enhance libvirt driver to configure guests based on
           the flavour parameter for page sizes

Partial-Implement: blueprint virt-driver-large-pages
Change-Id: I87e122a31f91448b203ba9201668f9143bf24752
Merge "libvirt: enhance config memory backing to handle hugepages"