For now, if only '--wipe-storage' is assigned, user can undefine a
domain normally. But actually '--wipe-storage' doesn't do anything,
and this may confuse user. Better is to require that '--wipe-storage'
only works if the user specifies volumes to be removed.

Before:
$ virsh undefine virt-tests-vm1 --wipe-storage
Domain virt-tests-vm1 has been undefined

After:
$ virsh undefine virt-tests-vm1 --wipe-storage
error: '--wipe-storage' requires '--storage <string>' or '--remove-all-storage'
Signed-off-by: NLi Yang <liyang.fnst@cn.fujitsu.com>
Signed-off-by: NEric Blake <eblake@redhat.com>