Renaming min_guarantee to balloon_min

Change-Id: I0c67e3830c7da3d99f3307a3d4f4e2af39853ea8
Signed-off-by: Doron Fediuck <dfediuck@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17029
Reviewed-by: Martin Sivák <msivak@redhat.com>
Tested-by: Martin Sivák <msivak@redhat.com>
Reviewed-by: Adam Litke <agl@us.ibm.com>