virtio_vdpa: setup correct vq size with callbacks get_vq_num_{max,min}

virtio_vdpa: setup correct vq size with callbacks get_vq_num_{max,min}

For the devices which implement the get_vq_num_min callback, the driver
should not negotiate with virtqueue size with the backend vdpa device if
the value returned by get_vq_num_min equals to the value returned by
get_vq_num_max.
This is useful for vdpa devices based on legacy virtio specfication.

Signed-off-by: Wu Zongyong <wuzongyong@linux.alibaba.com>
Link: https://lore.kernel.org/r/bc0551cec6c3f3dd9424b678b7c22d882aebab3a.1635493219.git.wuzongyong@linux.alibaba.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>