This patch allows the user-space to retrieve the MSI reserved region
requirements, if any. The implementation is based on capability chains,
now also added to VFIO_IOMMU_GET_INFO.

The returned info comprises the size and the alignment requirements

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>

---
v13 -> v14:
- new capability struct
- change the padding in vfio_iommu_type1_info

v11 -> v12:
- 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