vdpa: make get_vq_group and set_group_asid optional

This patch makes get_vq_group and set_group_asid optional. This is
needed to unbreak the vDPA parent that doesn't support multiple
address spaces.

Cc: Gautam Dawar <gautam.dawar@xilinx.com>
Fixes: aaca8373 ("vhost-vdpa: support ASID based IOTLB API")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20220609041901.2029-1-jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>