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>
Merge pull request #196 from liyangfnst/start_fix

libvirt.test: Fix a bug in start test