pcnet: make subsystem vendor id match hardware

Real pcnet device (AT2450) apparently has subsystem
device and vendor id set to 0, this is out of spec
(which requires that vendor id is obtained from PCI SIG)
but windows xp driver seems to need this in order
to associate.

qemu sets pci subsystem id to qumranet/qemu
since d350d97d196a632b6c7493acf07a061017fc6f7d,
debian does not yet have this patch.

https://bugzilla.redhat.com/show_bug.cgi?id=521247

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Anthony Liguori <aliguori@us.ibm.com>
