vfio: Refactor dma APIs for emulated devices

vfio: Refactor dma APIs for emulated devices

To use group helpers instead of opening group related code in the
API. This prepares moving group specific code out of vfio_main.c.

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: Lixiao Yang <[email protected]>
Tested-by: Yu He <[email protected]>
Signed-off-by: Yi Liu <[email protected]>
Signed-off-by: Nicolin Chen <[email protected]>
Signed-off-by: Jason Gunthorpe <[email protected]>