PCI: manual for SR-IOV user and driver developer

Reviewed-by: Randy Dunlap <rdunlap@xenotime.net>
Reviewed-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Yu Zhao <yu.zhao@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>