Description of problem:
An error is raised when trying to update a qos rule associated to a FIP that is attached to a running VM.

(overcloud) [stack@undercloud-0 tempest-dir]$ openstack network qos rule set --max-burst-kbits 800 --max-kbps 800 bw-lim-pol 8c8cffac-bcd4-4575-a667-5a000f402027
Failed to set Network QoS rule ID "8c8cffac-bcd4-4575-a667-5a000f402027": HttpException: 500: Server Error for url: http://10.0.0.135:9696/v2.0/qos/policies/7773994d-6209-47e7-a495-38401ee4d86b/bandwidth_limit_rules/8c8cffac-bcd4-4575-a66
7-5a000f402027, Request Failed: internal server error while processing your request.                                    


The following error can be found on neutron server logs:
2021-02-22 17:28:13.473 28 ERROR neutron.services.qos.drivers.manager [req-cb6cefd8-5456-4b89-b80c-511eb02643b3 9321be76b0eb49efac41a803459b4df0 8cfca0a542324d07ae2769ea4fb74c43 - default default] Extension driver 'OVNQosDriver' failed in
 update_policy: AttributeError: 'str' object has no attribute 'fip_id'                                                                                                                                                                       
2021-02-22 17:28:13.473 28 ERROR neutron.services.qos.drivers.manager Traceback (most recent call last):                                                                                                                                     
2021-02-22 17:28:13.473 28 ERROR neutron.services.qos.drivers.manager   File "/usr/lib/python3.6/site-packages/neutron/services/qos/drivers/manager.py", line 105, in call                                                                   
2021-02-22 17:28:13.473 28 ERROR neutron.services.qos.drivers.manager     getattr(driver, method_name)(*args, **kwargs)                                                                                                                      
2021-02-22 17:28:13.473 28 ERROR neutron.services.qos.drivers.manager   File "/usr/lib/python3.6/site-packages/networking_ovn/ml2/qos_driver.py", line 62, in update_policy                                                                  
2021-02-22 17:28:13.473 28 ERROR neutron.services.qos.drivers.manager     self._driver._ovn_client._qos_driver.update_policy(context, policy)                                                                                                
2021-02-22 17:28:13.473 28 ERROR neutron.services.qos.drivers.manager   File "/usr/lib/python3.6/site-packages/networking_ovn/common/client_extensions/qos.py", line 337, in update_policy                                                   
2021-02-22 17:28:13.473 28 ERROR neutron.services.qos.drivers.manager     fip_binding.fip_id)                                                                                                                                                
2021-02-22 17:28:13.473 28 ERROR neutron.services.qos.drivers.manager AttributeError: 'str' object has no attribute 'fip_id'                                                                                                                 



The rule is correctly updated in the neutron db (800 kbps is the new value):
(overcloud) [stack@undercloud-0 ~]$ openstack network qos rule list bw-lim-pol
+--------------------------------------+--------------------------------------+-----------------+----------+-----------------+----------+-----------+-----------+
| ID                                   | QoS Policy ID                        | Type            | Max Kbps | Max Burst Kbits | Min Kbps | DSCP mark | Direction |
+--------------------------------------+--------------------------------------+-----------------+----------+-----------------+----------+-----------+-----------+
| 8c8cffac-bcd4-4575-a667-5a000f402027 | 7773994d-6209-47e7-a495-38401ee4d86b | bandwidth_limit |      800 |             800 |          |           | egress    |
+--------------------------------------+--------------------------------------+-----------------+----------+-----------------+----------+-----------+-----------+


However, the OVN NBDB is not is not updated:
[root@controller-0 ~]# ovn-nbctl list qos f5e4c34b-3eee-4d8a-9792-06a3b85897d1
_uuid               : f5e4c34b-3eee-4d8a-9792-06a3b85897d1
action              : {}
bandwidth           : {burst=500, rate=500}
direction           : from-lport
external_ids        : {"neutron:fip_id"="9f24c1ea-151a-42a9-8ae9-49eee311fd52"}
match               : "inport == \"2b47155d-dbfe-4d13-b499-0cf9f7a146f2\" && ip4.src == 10.0.0.245 && is_chassis_resident(\"cr-lrp-2b47155d-dbfe-4d13-b499-0cf9f7a146f2\")"
priority            : 2002



Version-Release number of selected component (if applicable):
RHOS-16.1-RHEL-8-20210216.n.1

How reproducible:
100%

Steps to Reproduce:
1) Create server with a FIP attached to it
2) Create QoS policy with a QoS rule and associate it to the FIP
$ openstack network qos policy create bw-lim-pol
$ openstack network qos rule create bw-lim-pol --type bandwidth-limit --max-burst-kbits 500 --max-kbps 500 --egress                                                                                                                     
$ openstack floating ip set --qos-policy bw-lim-pol 10.0.0.245
3) Update the QoS rule
$ openstack network qos rule set --max-burst-kbits 800 --max-kbps 800 bw-lim-pol 8c8cffac-bcd4-4575-a667-5a000f402027