Intel VT-d 3.0 introduces scalable mode, and it has a bunch of capabilities
related to scalable mode translation, thus there are multiple combinations.
While this vIOMMU implementation wants simplify it for user by providing
typical combinations. User could config it by "x-scalable-mode" option. The
usage is as below:

"-device intel-iommu,x-scalable-mode=["legacy"|"modern"|"off"]"

 - "legacy": gives support for SL page table
 - "modern": gives support for FL page table, pasid, virtual command
 - "off": no scalable mode support
 -  if not configured, means no scalable mode support, if not proper
    configured, will throw error

Note: this patch is supposed to be merged when the whole vSVA patch series
were merged.

rfcv10 -> rfcv11:
*) this series uses /dev/ioasid for PASID allocation/free. In this patch,
   /dev/ioasid is opened it when deciding config.

rfcv5 (v2) -> rfcv6:
*) reports want_nested to VFIO;
*) assert iommu_set/unset_iommu_context() if vIOMMU is not scalable modern.

Cc: Kevin Tian <kevin.tian@intel.com>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Yi Sun <yi.y.sun@linux.intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Liu Yi L <yi.l.liu@intel.com>
Signed-off-by: Yi Sun <yi.y.sun@linux.intel.com>