virt-libvirt: Add a testcase for virsh change-media

When a guest is running, insert/update a file without
source path with "--config", then shutdown/destroy
guest, at this time, the guest cannot be started.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
virt-libvirt: Add floppy test for virsh change-media

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Merge pull request #788 from liyangfnst/change_media_floppy

virt-libvirt: Add floppy test for virsh change-media