libvirt.test: Fix hide bug in volume application test

Without this modification, when the first case is over,
the domain is still running, and it will cause failure
on next case.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Merge pull request #267 from liyangfnst/vol_app_fix

libvirt.test: Fix hide bug in volume application test