VT-d: fix and improve print_vtd_entries()

VT-d: fix and improve print_vtd_entries()

Fix leaking of mapped domain pages (root_entry and ctxt_entry when
falling out of the level traversing loop). Do this by re-arranging
things slightly so that a mapping is retained only as long as it
really is needed.

Fix the failure to use map_domain_page() in the level traversing loop
of the function.

Add a mssing return statement in one of the error paths.

Also I wonder whether not being able to call print_vtd_entries() from
iommu_page_fault_do_one() in ix86 is still correct, now that
map_domain_page() is IRQ safe.

Signed-off-by: Jan Beulich <jbeulich@novell.com>