vfio: Move vfio group specific code into group.c

vfio: Move vfio group specific code into group.c

This prepares for compiling out vfio group after vfio device cdev is
added. No vfio_group decode code should be in vfio_main.c, and neither
device->group reference should be in vfio_main.c.

No functional change is intended.

Link: https://lore.kernel.org/r/20221201145535.589687-11-yi.l.liu@intel.com
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Tested-by: Yu He <yu.he@intel.com>
Tested-by: Lixiao Yang <lixiao.yang@intel.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>