Description
libvirt should fail migration and provide error msg when migration from a newer host to an older.

Version:
rhel6.5:libvirt-0.10.2-29.el6_5.2.x86_64
rhel7.0:libvirt-1.1.1-14.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1:prepare two host,on with rhel6.5, the other is rhel7.0
2:prepare a healthy guest on rhel6.5 host
3:migrate the guest to rhel7.0, it can worked well.
4:migrate the guest back to rhel6.5
# virsh migrate --live rhel6 qemu+ssh://10.66.84.16/system --verbose
root.84.16's password:
Migration: [100 %]

it seems worked, but actually, qemu-kvm can't support this, after migration, the guest can't be used.
so libvirt should forbid this action, and provide some error msg to customer.


Actual results：


Expected results:


Additional info: