virtio: drop vp_legacy_set_queue_size

There's actually no way to set queue size on legacy virtio pci.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20220815220447.155860-1-mst@redhat.com>

virtio-pci: introduce legacy device module

Split common codes from virtio-pci-legacy so vDPA driver can reuse it
later.

Signed-off-by: Wu Zongyong <wuzongyong@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/71605acde5e97fcb2760a6973e406279fb1bbd33.1635493219.git.wuzongyong@linux.alibaba.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>