virpci: support driver_override sysfs interface

virpci: support driver_override sysfs interface

libvirt uses the new_id PCI sysfs interface to bind a PCI stub driver
to a PCI device. The new_id interface is known to be buggy and racey,
hence a more deterministic interface was introduced in the 3.12 kernel:
driver_override. For more details see

https://www.redhat.com/archives/libvir-list/2016-June/msg02124.html

For more details about the driver_override interface and examples of
its usage, see

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/drivers/pci/pci-driver.c?h=v3.12&id=782a985d7af26db39e86070d28f987cad21313c0

This patch adds support for the driver_override interface by

- adding new virPCIDevice{BindTo,UnbindFrom}StubWithOverride functions
  that use the driver_override interface
- renames the existing virPCIDevice{BindTo,UnbindFrom}Stub functions
  to virPCIDevice{BindTo,UnbindFrom}StubWithNewid to perserve existing
  behavior on new_id interface
- changes virPCIDevice{BindTo,UnbindFrom}Stub function to call one of
  the above depending on availability of driver_override

The patch includes a bit of duplicate code, but allows for easily
dropping the new_id code once support for older kernels is no
longer desired.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>