x86/NPT: don't walk page tables when changing types on a range

x86/NPT: don't walk page tables when changing types on a range

This builds on the fact that in order for no NPF VM exit to occur,
_PAGE_USER must always be set. I.e. by clearing the flag we can force a
VM exit allowing us to do similar lazy type changes as on EPT.

That way, the generic entry-wise code can go away, and we could remove
the range restriction in enforced on HVMOP_track_dirty_vram for XSA-27.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>