gluster: fix integer overflow error in rebalance status

Provides rebalance status values as strings to avoid overflow error
when a rebalance status values exceeds the XML-RPC limits

Change-Id: Iec44c47268318bcc105c00c2de0cf483012d3723
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1012393
Signed-off-by: Timothy Asir <tjeyasin@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19863
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>