The memory_region_tb_read tracepoint is unreachable, since notdirty
is supposed to apply only to writes.  The memory_region_tb_write
tracepoint is mis-named, because notdirty is not only used for TB
invalidation.  It is also used for e.g. VGA RAM updates and migration.

Replace memory_region_tb_write with memory_notdirty_write_access,
and place it in memory_notdirty_write_prepare where it can catch
all of the instances.  Add memory_notdirty_set_dirty to log when
we no longer intercept writes to a page.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>