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

mainline inclusion
from mainline-v5.19-rc1
commit a0c95f20
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5WXCZ
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a0c95f201170bd559737d3cdc8a950aea62f29c6



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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>
Signed-off-by: Pengyuan Zhao <zhaopengyuan@hisilicon.com>