hardware: introduce VirtNUMATopologyCellInstance

Introduces a new class VirtNUMATopologyCellInstance which extends
VirtNUMATopologyCell used to handle cells of VirtNUMAInstanceTopology
Also this commit make VirtNUMATopologyCell an abstract class.

Work-item: Enhance libvirt driver to report available
           large pages per NUMA node in the host state data

Change-Id: Icac19c17caec98ae31c039a2b22a7c1de546d9a2
Partial-Implement: blueprint virt-driver-large-pages
Merge "hardware: introduce VirtNUMATopologyCellInstance"