Make usage_from_instances consider current usage

VirtNUMAHostTopology.usage_from_instances will now consider the current
usage of the passed host, and also accepts a 'free' keyword argument,
which if set to True, will cause the instance usage to be taken out of
the considered host. Also adds better handling of  None and emtpy
params.

Blueprint: virt-driver-numa-placement
Change-Id: I62c59b5e1dcbd4db247282f170c6f2f501bafd3d
Merge "Make usage_from_instances consider current usage"