tools: xenstored: if the reply is too big then send E2BIG error

tools: xenstored: if the reply is too big then send E2BIG error

This fixes the issue for both C and ocaml xenstored, however only the ocaml
xenstored is vulnerable in its default configuration.

Adding a new error appears to be safe, since bit libxenstore and the Linux
driver at least treat an unknown error code as EINVAL.

This is XSA-72 / CVE-2013-4416.

Original ocaml patch by Jerome Maloberti <jerome.maloberti@citrix.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Thomas Sanders <thomas.sanders@citrix.com>