PCI/IOV: Factor out sriov_add_vfs()

Provide sriov_add_vfs() as a wrapper to scan for VFs that cleans up after
itself.  This is just a code simplification.  No functional change.

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