I don't think this is doable on the 5K side.
One tool that could be used on other platforms would be a policer. In this case a policer would be applied to each port to drop any Vmotion traffic over 2Gbps. Of course that would just be on an interface by interface basis and not the VLAN as a whole (i.e. two different hosts can be sending 2Gbps for a total of 4Gbps of Vmotion traffic on that VLAN). But I'm 95% sure the Nexus 5K can not do policing, even in version 5.0.
Another option would be to deal with this with traffic shapping on your virtual switches, but again that would be per link.
I'm assuming your using 10Gb links, if you are using 1Gb links, then I don't think there is a way to limit the traffic in the way you would like.
Joe