vfio: Fold VFIO_GROUP_GET_DEVICE_FD into vfio_group_get_device_fd()

No reason to split it up like this, just have one function to process the
ioctl.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/5-v2-0f9e632d54fb+d6-vfio_ioctl_split_jgg@nvidia.com


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