Introduce 64 bit integer write interface to xenstore

Introduce 64 bit integer write interface to xenstore

The current implementation of xen_backend only provides 32 bit integer
functions to write to xenstore. This patch adds two functions that
allow writing 64 bit integers (one generic function and another for
the backend only).

This patch also fixes the size of the char arrays used to represent
these integers as strings (originally 32 bytes, however no more than
12 bytes are needed for 32 bit integers and no more than 21 bytes are
needed for 64 bit integers).

Signed-off-by: Felipe Franciosi <felipe@paradoxo.org>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>