vfio: Export the device set open count

The open count of a device set is the sum of the open counts of all
devices in the set. Drivers can use this value to determine whether
shared resources are in use without tracking them manually or accessing
the private open_count in vfio_device.

Signed-off-by: Anthony DeRossi <ajderossi@gmail.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Link: https://lore.kernel.org/r/20221110014027.28780-3-ajderossi@gmail.com


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>