iommu: Move dom0 setup code to __hwdom_init

iommu: Move dom0 setup code to __hwdom_init

When the hardware domain is split from domain 0, the initialization code
for the hardware domain cannot be in the __init section, since the
actual domain creation happens after these sections have been discarded.
Create a __hwdom_init section designator to annotate these functions,
and control it using the XSM configuration option for now (since XSM is
required to take advantage of the security benefits of disaggregation).

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>