libvirt: un-cruft _get_guest_numa_config

Several issues get cleaned up with this patch - all of which will make
adding CPU pinning into the mix a lot cleaner.

1) Unecessary context argument to the method is dropped.

2) Secondly - we pass in the InstanceNUMATopology object (or None)
instead of passing in the whole instance and then getting the topology
out - this is useful as we will need instance NUMA for generating topology
as well, and this avoids repetitive code in a lot of places.

3) Add missing tests for updated method the method
get_cpu_numa_config_from_instance

Change-Id: I829465d1612f42e7af92927584b31ca6a2349d5a
Merge "libvirt: un-cruft _get_guest_numa_config"