tools/oxenstored: mkdir conflicts were sometimes missed

tools/oxenstored: mkdir conflicts were sometimes missed

Due to how set_write_lowpath was used here it didn't detect create/delete
conflicts.  When we create an entry we must mark our parent as modified
(this is what creating a new node via write does).

Otherwise we can have 2 transactions one creating, and another deleting a node
both succeeding depending on timing.  Or one transaction reading an entry,
concluding it doesn't exist, do some other work based on that information and
successfully commit even if another transaction creates the node via mkdir
meanwhile.

Signed-off-by: Edwin Török <edvin.torok@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>