vdpa: introduce virtio pci driver

This patch introduce a vDPA driver for virtio-pci device. It bridges
the virtio-pci control command to the vDPA bus. This will be used for
features prototyping and testing.

Note that get/restore virtqueue state is not supported which needs
extension on the virtio specification.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210223061905.422659-4-jasowang@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>