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 <kevin.tian@intel.com>
CC: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Eric Auger <eric.auger@redhat.com>
Cc: Jean-Philippe Brucker <jean-philippe@linaro.org>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Suggested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Liu Yi L <yi.l.liu@intel.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>