vDPA/ifcvf: get_max_vq_size to return max size

Since we already implemented vdpa_config_ops.get_vq_size,
so get_max_vq_size can return the acutal max size of the
virtqueues other than the max allowed safe size.

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Message-Id: <20240202163905.8834-10-lingshan.zhu@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>