vdpa: set dma mask for vDPA device

Setting DMA mask for vDPA device in case that there are virtqueue that
is not backed by DMA so the vDPA device could be advertised as the DMA
device that is used by DMA API for software emulated virtqueues.

Reviewed-by: Eli Cohen <elic@nvidia.com>
Tested-by: Eli Cohen <elic@nvidia.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20230119061525.75068-5-jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>