virtio-pci: Don't force Subsystem Vendor ID = Vendor ID

The statement being removed doesn't change anything as virtio PCI devices already
have Subsystem Vendor ID set to pci_default_sub_vendor_id (0x1af4), same as Vendor
ID. And the Virtio spec does not require the two to be equal, either:

  "The PCI Subsystem Vendor ID and the PCI Subsystem Device ID MAY reflect the PCI
  Vendor and Device ID of the environment (for informational purposes by the driver)."

Background:

Following the recent virtio-win licensing change, several vendors are planning to
ship their own certified version of Windows guest Virtio drivers, potentially taking
advantage of Windows Update as a distribution channel. It is therefore critical that
each vendor uses their own PCI Subsystem Vendor ID for Virtio devices to prevent
drivers from other vendors binding to it.

This would be trivially done by adding:

  k->subsystem_vendor_id = ...

to virtio_pci_class_init(). Except for the problematic statement deleted by this
patch, which reverts the Subsystem Vendor ID back to 0x1af4 for legacy devices for
no good reason.

Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>