Do you know if you are using "Beacon probing" or "Link State" on your vSwitches as network failure detection?
Ordinary vMotion should not generate much broadcasts * and should not in normal situations not be seen as flapping by the physical switches.
* 10 broadcasts per vMotion is sent, with fakes broadcasts from the vmkernel pretending to be the VM: http://rickardnobel.se/vswitch-notify-switches-setting