spmstop: add "--" to /usr/bin/kill invocation parameters

In Ubuntu kill, invocation like "kill -9 -xxx" is not parsed correctly.
We should use "kill -9 -- -xxx" instead.

The patch is on going work of porting VDSM to Ubuntu
http://www.ovirt.org/VDSM_on_Ubuntu

Change-Id: I4746290d4b6c8130c3ecd94f1ef6bd53689b991f
Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/12817
Reviewed-by: Dan Kenigsberg <danken@redhat.com>