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

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

This requires a new P2M backend hook and a little bit of extra care on
accounting in the generic function.

Note that even on leaf entries we must not immediately set the new
type (in an attempt to avoid the EPT_MISCONFIG VM exits), since the
global accounting in p2m_change_type_range() gets intentionally done
only after updating page tables (or else the update there would
conflict with the function's own use of p2m_is_logdirty_range()), and
the correct type can only be calculated with that in place.

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