vfio: Split up vfio_group_get_device_fd()

The split follows the pairing with the destroy functions:

 - vfio_group_get_device_fd() destroyed by close()

 - vfio_device_open() destroyed by vfio_device_fops_release()

 - vfio_device_assign_container() destroyed by
   vfio_group_try_dissolve_container()

The next patch will put a lock around vfio_device_assign_container().

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Tested-by: Matthew Rosato <mjrosato@linux.ibm.com>
Link: https://lore.kernel.org/r/3-v2-d035a1842d81+1bf-vfio_group_locking_jgg@nvidia.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>