oxenstored: Enforce a maximum message size of 4096 bytes

oxenstored: Enforce a maximum message size of 4096 bytes

The maximum size of a message is part of the protocol spec in
  xen/include/public/io/xs_wire.h

Before this patch a client which sends an overly large message can
cause a buffer read overrun.

Note if a badly-behaved client sends a very large message
then it will be difficult for them to make their connection
work again-- they will probably need to reboot.

This is a security issue, part of XSA-38 / CVE-2013-0215.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>
xen-unstable changeset: 26522:ffd30e7388ad
Backport-requested-by: security@xen.org
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>