QoS SR-IOV: allow to reset vf rate when VF is assigend to VM

QoS SR-IOV: allow to reset vf rate when VF is assigend to VM

delete_bandwidth_limit method is called in 2 scenarios:
1. when deleting a port
2. when removing a policy from a port

self.eswitch_mgr.clear_max_rate is valdating if VF is not assigned to VM
which is good for case 1, but prevent reseting the max rate for case 2.
for case 2 the delete_bandwidth_limit should  call _set_vf_max_rate with max_kbps=0
to reset VF max rate for assigned VM.

Closes-Bug: #1504165

Change-Id: Ief708531708b8faa918603298d49d39ef5ba2df5