vfio/migration: Fail adding device with enable-migration=on and exist…

…ing blocker

If a device with enable-migration=on is added and it causes a migration
blocker, adding the device should fail with a proper error.

This is not the case with multiple device migration blocker when the
blocker already exists. If the blocker already exists and a device with
enable-migration=on is added which causes a migration blocker, adding
the device will succeed.

Fix it by failing adding the device in such case.

Fixes: 8bbcb64 ("vfio/migration: Make VFIO migration non-experimental")
Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>

hw/vfio/common.c

@@ -394,8 +394,7 @@ int vfio_block_multiple_devices_migration(VFIODevice *vbasedev, Error **errp)
 {
     int ret;
 
-    if (multiple_devices_migration_blocker ||
-        vfio_multiple_devices_migration_is_supported()) {
+    if (vfio_multiple_devices_migration_is_supported()) {
         return 0;
     }
 
@@ -405,6 +404,10 @@ int vfio_block_multiple_devices_migration(VFIODevice *vbasedev, Error **errp)
         return -EINVAL;
     }
 
+    if (multiple_devices_migration_blocker) {
+        return 0;
+    }
+
     error_setg(&multiple_devices_migration_blocker,
                "Multiple VFIO devices migration is supported only if all of "
                "them support P2P migration");