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/[email protected]
Reviewed-by: Kevin Tian <[email protected]>
Reviewed-by: Jason Gunthorpe <[email protected]>
Reviewed-by: Alex Williamson <[email protected]>
Tested-by: Yu He <[email protected]>
Tested-by: Lixiao Yang <[email protected]>
Signed-off-by: Yi Liu <[email protected]>
Signed-off-by: Jason Gunthorpe <[email protected]>

Author: yiliu1765

drivers/vfio/Makefile

@@ -4,6 +4,7 @@ vfio_virqfd-y := virqfd.o
 obj-$(CONFIG_VFIO) += vfio.o
 
 vfio-y += vfio_main.o \
+	  group.o \
 	  iova_bitmap.o
 vfio-$(CONFIG_IOMMUFD) += iommufd.o
 vfio-$(CONFIG_VFIO_CONTAINER) += container.o