libvirt: Add 2 new test cases for virsh change-media

1.When a guest is shutoff, user can insert/update a source file
  without source path, but the guest cannot be started
2.When a guest is started, user cannot insert/update a source file
  without source path.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Merge pull request #738 from liyangfnst/change_media_new_test

libvirt: Add 2 new test cases for virsh change-media