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>
diff --git a/qerror.c b/qerror.c
index 4b48b39..8e30e2d 100644
--- a/qerror.c
+++ b/qerror.c
@@ -73,6 +73,10 @@
         .desc      = "Device '%(device)' is in use",
     },
     {
+        .error_fmt = QERR_DEVICE_FEATURE_BLOCKS_MIGRATION,
+        .desc      = "Migration is disabled when using feature '%(feature)' in device '%(device)'",
+    },
+    {
         .error_fmt = QERR_DEVICE_LOCKED,
         .desc      = "Device '%(device)' is locked",
     },