hw/virtio: Add PCIe capability to virtio devices

The virtio devices are converted to PCI-Express
if they are plugged into a PCI-Express bus and
the 'modern' protocol is enabled.

Devices plugged directly into the Root Complex as
Integrated Endpoints remain PCI.

Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>