x86/CPUID: unconditionally set XEN_HVM_CPUID_IOMMU_MAPPINGS

x86/CPUID: unconditionally set XEN_HVM_CPUID_IOMMU_MAPPINGS

This is a revert of f5cfa0985673 plus a rework of the comment that
accompanies the setting of the flag so we don't forget why it needs to
be unconditionally set: it's indicating whether the version of Xen has
the original issue fixed and IOMMU entries are created for
grant/foreign maps.

If the flag is only exposed when the IOMMU is enabled the guest could
resort to use bounce buffers when running backends as it would assume
the underlying Xen version still has the bug present and thus
grant/foreign maps cannot be used with devices.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>