vfio/type1: Add domain at(de)taching group helpers

This adds helpers to attach or detach a domain to a
group. This will replace iommu_attach_group() which
only works for non-mdev devices.

If a domain is attaching to a group which includes the
mediated devices, it should attach to the iommu device
(a pci device which represents the mdev in iommu scope)
instead. The added helper supports attaching domain to
groups for both pci and mdev devices.

Cc: Ashok Raj <ashok.raj@intel.com>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Sanjay Kumar <sanjay.k.kumar@intel.com>
Signed-off-by: Liu Yi L <yi.l.liu@intel.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>