PCI: Expose SR-IOV offset, stride, and VF device ID via sysfs

Expose the SR-IOV device offset, stride, and VF device ID via sysfs to make
it easier for userspace applications to consume them.

Signed-off-by: Filippo Sironi <sironi@amazon.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>