Description of problem:
Block based migration doesn't work for instances that have a volume attached. When trying to block migrate an instance that has a cinder volume attached the following error shows up:
MigrationError: Migration error: Cannot block migrate instance 7dfe54c2-1b44-4fcd-892e-36fcb68a387c with mapped volumes

The volumes are placed on a shared NFS storage. 

Version-Release number of selected component (if applicable):
openstack-nova-novncproxy-2015.1.2-13.el7ost.noarch
openstack-nova-console-2015.1.2-13.el7ost.noarch
python-nova-2015.1.2-13.el7ost.noarch
openstack-nova-conductor-2015.1.2-13.el7ost.noarch
openstack-nova-common-2015.1.2-13.el7ost.noarch
openstack-nova-compute-2015.1.2-13.el7ost.noarch
openstack-nova-cert-2015.1.2-13.el7ost.noarch
openstack-nova-scheduler-2015.1.2-13.el7ost.noarch
openstack-nova-api-2015.1.2-13.el7ost.noarch
python-novaclient-2.23.0-2.el7ost.noarch

openstack-tripleo-heat-templates-0.8.6-112.el7ost.noarch

How reproducible:
100%

Steps to Reproduce:
1. Deploy overcloud with 3 controllers and 2 computes 
2. Deploy instance and attach a cinder volume to it
3. Block migrate the instance to the other hypervisor:
nova live-migration --block-migrate vm01 overcloud-compute-1.localdomain

Actual results:
 MigrationError: Migration error: Cannot block migrate instance 7dfe54c2-1b44-4fcd-892e-36fcb68a387c with mapped volumes

Expected results:
The instance is migrated.

Additional info:
This is the block_migration_flag set in nova.conf:
#block_migration_flag=VIR_MIGRATE_UNDEFINE_SOURCE, VIR_MIGRATE_PEER2PEER, VIR_MIGRATE_LIVE, VIR_MIGRATE_TUNNELLED, VIR_MIGRATE_NON_SHARED_INC