x86/HVM: correct page dirty marking in hvm_map_guest_frame_rw()

x86/HVM: correct page dirty marking in hvm_map_guest_frame_rw()

Rather than dirtying a page when establishing a (permanent) mapping,
dirty it when the page gets unmapped, or - if still mapped - on the
final iteration of a save operation (or in other cases where the guest
is paused or already shut down). (Transient mappings continue to get
dirtied upon getting mapped, to avoid the overhead of tracking.)

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>