VT-d: free all-empty page tables

VT-d: free all-empty page tables

When a page table ends up with no present entries left, it can be
replaced by a non-present entry at the next higher level. The page table
itself can then be scheduled for freeing.

Note that while its output isn't used there yet,
pt_update_contig_markers() right away needs to be called in all places
where entries get updated, not just the one where entries get cleared.

Note further that while pt_update_contig_markers() updates perhaps
several PTEs within the table, since these are changes to "avail" bits
only I do not think that cache flushing would be needed afterwards. Such
cache flushing (of entire pages, unless adding yet more logic to me more
selective) would be quite noticable performance-wise (very prominent
during Dom0 boot).

Also note that cache sync-ing is likely more strict than necessary. This
is both to be on the safe side as well as to maintain the pattern of all
updates of (potentially) live tables being accompanied by a flush (if so
needed).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Paul Durrant <paul@xen.org>