gluster: use rebalance cli xml output

glusterVolumeRebalanceStart and glusterVolumeRebalanceStop use gluster
cli xml output than stdout parsing.

glusterVolumeRebalanceStart is not consumed by engine/RHS-C yet, its
OK to differ in compatibility issue now.

Change-Id: I4c3f1328245fd8fb2338d03cec477ea059f3b389
Signed-off-by: Bala.FA <barumuga@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13003
Reviewed-by: Timothy Asir <tjeyasin@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>