vfio/spapr: Introduce a sPAPR VFIOIOMMU QOM interface

vfio/spapr: Introduce a sPAPR VFIOIOMMU QOM interface

Move vfio_spapr_container_setup() to a VFIOIOMMUClass::setup handler
and convert the sPAPR VFIOIOMMUOps struct to a QOM interface. The
sPAPR QOM interface inherits from the legacy QOM interface because
because both have the same basic needs. The sPAPR interface is then
extended with the handlers specific to the sPAPR IOMMU.

This allows reuse and provides better abstraction of the backends. It
will be useful to avoid compiling the sPAPR IOMMU backend on targets
not supporting it.

Reviewed-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Tested-by: Eric Farman <farman@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>