libvirt.test: Fix a bug in start test

Perhaps there'is a process 'sleep 60' always exists, so the
original filter will fail to get the right sleep process.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
This will not solve the problem if there is another service "sleep 1000". 

I think we should store the pid of the process we created in guest and verify the result with ("ps %s|grep 'sleep 1000'|grep -v grep" % sleep_pid).