pci.c: Add opaque argument to pci_for_each_device.
The purpose is to have a more generic pci_for_each_device by passing an extra
argument to the function called on every device.
This patch will be used in a next patch.
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com> Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com> Acked-by: Michael S. Tsirkin <mst@redhat.com> Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>