Description of problem:
the data of luks disk can't be accessed after cross migrating guest from rhel7.5 host to rhel7.4 host

Version-Release number of selected component (if applicable):

source rhel7.5 host:
libvirt-3.9.0-6.el7.x86_64
qemu-kvm-rhev-2.10.0-13.el7.x86_64

destination rhel7.4.z host:
libvirt-3.2.0-14.el7_4.5.x86_64
qemu-kvm-rhev-2.9.0-16.el7_4.13.x86_64

How reproducible:
100%

Steps to Reproduce:

1.prepare the luks secret in both hosts
1) prepare secret xml file
[root@lmen1 ~]# cat secret-luks.xml
<secret ephemeral='no' private='yes'>
  <uuid>f981dd17-143f-45bc-88e6-ed1fe20ce9da</uuid>
  <description>LUKS Sample Secret</description>
  <usage type='volume'>
    <volume>/var/libvirt/images/luks.img</volume>
  </usage>
</secret>

2) define luks secret
[root@lmen1 ~]# virsh secret-define secret-luks.xml
Secret f981dd17-143f-45bc-88e6-ed1fe20ce9da created

3) set value of the luks secret.
[root@lmen1 ~]# MYSECRET=`printf %s "redhat" | base64`
[root@lmen1 ~]# virsh secret-set-value  f981dd17-143f-45bc-88e6-ed1fe20ce9da $MYSECRET
  Secret value set

2.start a guest in rhel7.5 host with xml:  -->***encryption is the sub-element of source***
 <disk type='file' device='disk'>
      <driver name='qemu' type='raw' cache='none'/>
      <source file='/var/lib/libvirt/images/luks.img'>
        <encryption format='luks'>
          <secret type='passphrase' uuid='f981dd17-143f-45bc-88e6-ed1fe20ce9da'/>
        </encryption>
      </source>
      <target dev='sdb' bus='scsi'/>
      <address type='drive' controller='0' bus='0' target='0' unit='1'/>
    </disk>

[root@lmen1 ~]# virsh start test
Domain test started

3.migrate the guest from rhel7.5 host to the rhel7.4.z destination  host
[root@lmen1 ~]# virsh migrate test --live  qemu+ssh://lmen/system


4.check the xml in the destination host:
[root@localhost ~]# virsh dumpxml test 
...
 <disk type='file' device='disk'>
      <driver name='qemu' type='raw' cache='none'/>
      <source file='/var/lib/libvirt/images/luks.img'/>  -->***there is not encryption element any more***
      <backingStore/>
      <target dev='sdb' bus='scsi'/>
      <alias name='scsi0-0-0-1'/>
      <address type='drive' controller='0' bus='0' target='0' unit='1'/>
    </disk>

...

login the guest in the destination host,
running "fdisk -l" can find the luks disk, but can't mount the disk:
#mount /dev/sda /mnt
mount:unknown filesystem type 'crypto_LUKS'

Actual results:
migration successfully，but can't access the data of the disk any more

Expected results:

If the customer migrate the guest with luks disk from rhel7.5 to rhel7.4,they will can not access the data after the migration successfully,and they also can not access the data if they migrate the guest back to rhel7.5 host(the encryption element still missing)

So I think the expected result is:
the migration should be prevented in the source rhel7.5 host

Additional info: