Fix example of failed migrarion for LVM->RBD
Fix example of failed migrarion for LVM->RBD

Migration from and to RBD devices works for in-use volumes since
Ussuri release. Also as of now a migration of in-use volumes for
non-active VMs is prohibited for any backends.

Related-Bug: #1673090
Related-Bug: #1868996
Closes-Bug: #1969913
Change-Id: Id639c412a6f5ad24ab9c80416c56c1d99b3f104c

mahak/cinder,mahak/cinder,openstack/cinder,openstack/cinder