iommu: don't need to map dom0 page when the PT is shared

iommu: don't need to map dom0 page when the PT is shared

Currently iommu_init_dom0 is browsing the page list and call map_page callback
on each page.

On both AMD and VTD drivers, the function will directly return if the page
table is shared with the processor. So Xen can safely avoid to run through
the page list.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>