The stats module is used to decide if the InstancePCIRequests of a boot
request can fit to a given compute host and to decide which PCI device
pool can fulfill the requests. It is used both during scheduling and also
during the PCI claim code.
PCI devices now modelled in placement and the allocation_candidate query
now requests PCI resources therefore each allocation candidate returned
from placement already restricts which PCI devices can be used during
the PciPassthroughFilter, the NumaTopologyFilter, and PCI claim code
paths. This patch adapts the stats module to consider the PCI
allocation candidate or the already made placement PCI allocation when
filtering the PCI device pools.