Support ioTune values >2^31 in getStats over xml-rpc

This fixes issue with getStats over xml-rpc when ioTune value
is accidentally increased beyond cca 2e9 (2GB per second).

Change-Id: If5005d7fdc5397df2480fa8d0c15b003a52e8626
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/30215
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>