ivshmem: use migration blockers to prevent live migration in peer mode (v2)

Now when you try to migrate with ivshmem, you get a proper QMP error:

(qemu) migrate tcp:localhost:1025
Migration is disabled when using feature 'peer mode' in device 'ivshmem'
(qemu)

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
