fix DOMID_IO mapping permission checks

fix DOMID_IO mapping permission checks

When the permission checks for memory mapping were moved from
get_pg_owner to xsm_mmu_update in aaba7a677, the exception for DOMID_IO
was not taken into account. This will cause IO memory mappings by PV
domains (mini-os in particular) to fail when XSM/FLASK is not being
used. This patch reintroduces the exception for DOMID_IO; the actual
restrictions on IO memory mappings have always been checked separately
using iomem_access_permitted, so this change should not break existing
access control.

Reported-by: Eduardo Peixoto Macedo <epm@cin.ufpe.br>
Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>