Oxenstored 32->31 bit integer truncation issues

ISSUE DESCRIPTION

Integers in Ocaml are 63 or 31 bits of signed precision.
The Ocaml Xenbus library takes a C uint32_t out of the ring and casts it directly to an Ocaml integer. In 64-bit Ocaml builds this is fine, but in 32-bit builds, it truncates off the most significant bit, and then creates unsigned/signed confusion in the remainder.
This in turn can feed a negative value into logic not expecting a negative value, resulting in unexpected exceptions being thrown.
The unexpected exception is not handled suitably, creating a busy-loop trying (and failing) to take the bad packet out of the xenstore ring.

IMPACT

A malicious or buggy guest can write a packet into the xenstore ring which causes 32-bit builds of oxenstored to busy loop.

VULNERABLE SYSTEMS

All versions of Xen are affected.
Systems running a 32-bit build of oxenstored are affected.
Systems running a 64-bit build of oxenstored, or systems running © xenstored are not affected.