PCI/IOV: Add flag so platforms can skip VF scanning

Provide a flag to skip scanning for new VFs after SR-IOV enablement.  This
can be set by implementations for which the VFs are already reported by
other means.

Signed-off-by: Sebastian Ott <sebott@linux.ibm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>