vfio/type1: Simplify bus_type determination

Since IOMMU groups are mandatory for drivers to support, it stands to
reason that any device which has been successfully added to a group
must be on a bus supported by that IOMMU driver, and therefore a domain
viable for any device in the group must be viable for all devices in
the group. This already has to be the case for the IOMMU API's internal
default domain, for instance. Thus even if the group contains devices on
different buses, that can only mean that the IOMMU driver actually
supports such an odd topology, and so without loss of generality we can
expect the bus type of any device in a group to be suitable for IOMMU
API calls.

Furthermore, scrutiny reveals a lack of protection for the bus being
removed while vfio_iommu_type1_attach_group() is using it; the reference
that VFIO holds on the iommu_group ensures that data remains valid, but
does not prevent the group's membership changing underfoot.

We can address both concerns by recycling vfio_bus_type() into some
superficially similar logic to indirect the IOMMU API calls themselves.
Each call is thus protected from races by the IOMMU group's own locking,
and we no longer need to hold group-derived pointers beyond that scope.
It also gives us an easy path for the IOMMU API's migration of bus-based
interfaces to device-based, of which we can already take the first step
with device_iommu_capable(). As with domains, any capability must in
practice be consistent for devices in a given group - and after all it's
still the same capability which was expected to be consistent across an
entire bus! - so there's no need for any complicated validation.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/194a12d3434d7b38f84fa96503c7664451c8c395.1656092606.git.robin.murphy@arm.com
[aw: add comment to vfio_iommu_device_capable()]
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

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