PCI: Enable unassigned resource reallocation on per-bus basis

pci_realloc_detect() turns on automatic resource allocation when it finds
unassigned SR-IOV resources.  Previously it did this on a global basis, so
we enabled reallocation if any PCI device anywhere had an unassigned SR-IOV
resource.

This patch changes pci_realloc_detect() so it looks at a single bus, so we
can do this when a host bridge is hot-added.

[bhelgaas: changelog]
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>