vhost-vdpa: introduce uAPI to get the number of address spaces

This patch introduces the uAPI for getting the number of address
spaces supported by this vDPA device.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Gautam Dawar <gdawar@xilinx.com>
Message-Id: <20220330180436.24644-13-gdawar@xilinx.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>