tools/oxenstored: Make evaluation order explicit

tools/oxenstored: Make evaluation order explicit

In Store.path_write(), Path.apply_modify() updates the node_created
reference and both the value of apply_modify() and node_created are
returned by path_write().

At least with OCaml 4.06.1 this leads to the value of node_created being
returned *before* it is updated by apply_modify().  This in turn leads
to the quota for a domain not being updated in Store.write().  Hence, a
guest can create an unlimited number of entries in xenstore.

The fix is to make evaluation order explicit.

This is XSA-272.

Signed-off-by: Christian Lindig <christian.lindig@citrix.com>
Reviewed-by: Rob Hoes <rob.hoes@citrix.com>