vfio/type1: Refactor vfio_iommu_type1_ioctl()

This patch refactors the vfio_iommu_type1_ioctl() to use switch instead of
if-else, and each command got a helper function.

Cc: Kevin Tian <[email protected]>
CC: Jacob Pan <[email protected]>
Cc: Alex Williamson <[email protected]>
Cc: Eric Auger <[email protected]>
Cc: Jean-Philippe Brucker <[email protected]>
Cc: Joerg Roedel <[email protected]>
Cc: Lu Baolu <[email protected]>
Reviewed-by: Eric Auger <[email protected]>
Suggested-by: Christoph Hellwig <[email protected]>
Signed-off-by: Liu Yi L <[email protected]>
Signed-off-by: Alex Williamson <[email protected]>

Author: yiliu1765