pci.c: Add opaque argument to pci_for_each_device.

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>