tools/ocaml/xenstored: Check for maxrequests before performing operations

tools/ocaml/xenstored: Check for maxrequests before performing operations

Previously we'd perform the operation, record the updated tree in the
transaction record, then try to insert a watchop path and the reply packet.

If we exceeded max requests we would've returned EQUOTA, but still:
* have performed the operation on the transaction's tree
* have recorded the watchop, making this queue effectively unbounded

It is better if we check whether we'd have room to store the operation before
performing the transaction, and raise EQUOTA there.  Then the transaction
record won't grow.

This is part of XSA-326 / CVE-2022-42317.

Signed-off-by: Edwin Török <edvin.torok@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>