PCI: SRIOV control and status via sysfs

Provide files under sysfs to determine the maximum number of VFs
an SR-IOV-capable PCIe device supports, and methods to enable and
disable the VFs on a per-device basis.

Currently, VF enablement by SR-IOV-capable PCIe devices is done
via driver-specific module parameters.  If not setup in modprobe files,
it requires admin to unload & reload PF drivers with number of desired
VFs to enable.  Additionally, the enablement is system wide: all
devices controlled by the same driver have the same number of VFs
enabled.  Although the latter is probably desired, there are PCI
configurations setup by system BIOS that may not enable that to occur.

Two files are created for the PF of PCIe devices with SR-IOV support:

    sriov_totalvfs	Contains the maximum number of VFs the device
			could support as reported by the TotalVFs register
			in the SR-IOV extended capability.

    sriov_numvfs	Contains the number of VFs currently enabled on
			this device as reported by the NumVFs register in
			the SR-IOV extended capability.

			Writing zero to this file disables all VFs.

			Writing a positive number to this file enables that
			number of VFs.

These files are readable for all SR-IOV PF devices.  Writes to the
sriov_numvfs file are effective only if a driver that supports the
sriov_configure() method is attached.

Signed-off-by: Donald Dutile <ddutile@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>