ocaml-qmp: Use float to represent message timestamps

On 32-bit platforms max_int is 2^30-1, so 32-bit Unix timestamps
can't be represented.   The OCaml standard library uses floats
to represent timestamps, so we will follow suit.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Merge pull request #1 from euanh/32bit-timestamp-fix

ocaml-qmp: Use float to represent message timestamps