vfio/mdev: Use sysfs_emit() to instead of sprintf()

Follow the advice of the Documentation/filesystems/sysfs.rst and show()
should only use sysfs_emit() or sysfs_emit_at() when formatting the
value to be returned to user space.

Signed-off-by: Bo Liu <liubo03@inspur.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20230129084117.2384-1-liubo03@inspur.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>