…ility chains

This patch allows the user-space to retrieve the MSI geometry. The
implementation is based on capability chains, now also added to
VFIO_IOMMU_GET_INFO.

The returned info comprise:
- whether the MSI IOVA are constrained to a reserved range (x86 case) and
  in the positive, the start/end of the aperture,
- or whether the IOVA aperture need to be set by the userspace. In that
  case, the size and alignment of the IOVA window to be provided are
  returned.

In case the userspace must provide the IOVA aperture, we currently report
a size/alignment based on all the doorbells registered by the host kernel.
This may exceed the actual needs.

Signed-off-by: Eric Auger <eric.auger@redhat.com>

---
v11 -> v11:
- msi_doorbell_pages was renamed msi_doorbell_calc_pages

v9 -> v10:
- move cap_offset after iova_pgsizes
- replace __u64 alignment by __u32 order
- introduce __u32 flags in vfio_iommu_type1_info_cap_msi_geometry and
  fix alignment
- call msi-doorbell API to compute the size/alignment

v8 -> v9:
- use iommu_msi_supported flag instead of programmable
- replace IOMMU_INFO_REQUIRE_MSI_MAP flag by a more sophisticated
  capability chain, reporting the MSI geometry

v7 -> v8:
- use iommu_domain_msi_geometry

v6 -> v7:
- remove the computation of the number of IOVA pages to be provisionned.
  This number depends on the domain/group/device topology which can
  dynamically change. Let's rely instead rely on an arbitrary max depending
  on the system

v4 -> v5:
- move msi_info and ret declaration within the conditional code

v3 -> v4:
- replace former vfio_domains_require_msi_mapping by
  more complex computation of MSI mapping requirements, especially the
  number of pages to be provided by the user-space.
- reword patch title

RFC v1 -> v1:
- derived from
  [RFC PATCH 3/6] vfio: Extend iommu-info to return MSIs automap state
- renamed allow_msi_reconfig into require_msi_mapping
- fixed VFIO_IOMMU_GET_INFO