iommu: fix check for autotranslated hardware domain

iommu: fix check for autotranslated hardware domain

The current position of the check_hwdom_reqs is wrong, as there's a
is_iommu_enabled at the top of the function that will prevent getting
to the check on systems without an IOMMU, because the hardware domain
won't have the XEN_DOMCTL_CDF_iommu flag set.

Move the position of the check so it's done before the
is_iommu_enabled one, and thus attempts to create a translated
hardware domain without an IOMMU can be detected.

Fixes: f89f555827a ('remove late (on-demand) construction of IOMMU page tables')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>