libvirt.test: Skip test if disk source is a volume

when execute undefine with '--remove-all-storage', the
associated storage volumes will be removed, it's dangerous
for domain.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Merge pull request #300 from liyangfnst/vda_delete

libvirt.test: Skip test if disk source is a volume