tools/xenstore: reduce number of watch events

tools/xenstore: reduce number of watch events

When removing a watched node outside of a transaction, two watch events
are being produced instead of just a single one.

When finalizing a transaction watch events can be generated for each
node which is being modified, even if outside a transaction such
modifications might not have resulted in a watch event.

This happens e.g.:

- for nodes which are only modified due to added/removed child entries
- for nodes being removed or created implicitly (e.g. creation of a/b/c
  is implicitly creating a/b, resulting in watch events for a, a/b and
  a/b/c instead of a/b/c only)

Avoid these additional watch events, in order to reduce the needed
memory inside Xenstore for queueing them.

This is being achieved by adding event flags to struct accessed_node
specifying whether an event should be triggered, and whether it should
be an exact match of the modified path. Both flags can be set from
fire_watches() instead of implying them only.

This is part of XSA-326.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
(cherry picked from commit 3a96013a3e17baa07410b1b9776225d1d9a74297)