up till libvirt 0.10.0, i.e. RHEL 6.3 there was a default maximum speed of migrating VM of 30Mbps.
vdsm relied on the default libvirt/qemu behavior.
This was changed in 6.4 nd so starting with RHEV-M 3.1.3 there is no limit enforced anymore. This can cause overload of network interfaces as the whole bandwidth is used in case of mass migration (either manual or induced by moving a host to maintenance mode) 

The fix would be to use the former 30Mbps value as the default in vdsm config.
something like:
-        ('migration_max_bandwidth', '0',
-            'Maximum bandwidth for migration, in mbps, 0 means libvirt\'s '
-            'default (30mbps?).'),
+        ('migration_max_bandwidth', '30',
+            'Maximum bandwidth for migration, in mbps, 0 means unlimited, former libvirt\'s '
+            'default is 30Mbps.'),

plus we need to fix upgrade as well, I guess