qemu: do not revert to NULL bandwidth

Otherwise an attempt to set an invalid value:
  virsh domiftune rhel8.2 vnet0 --outbound 4294968
on an interface with no bandwidth set crashes.
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Fixes: f02e21cb
https://bugzilla.redhat.com/show_bug.cgi?id=1800505Reviewed-by: NDaniel Henrique Barboza <danielhb413@gmail.com>

src/qemu/qemu_driver.c

@@ -11682,8 +11682,10 @@ qemuDomainSetInterfaceParameters(virDomainPtr dom,
                                                net->bandwidth,
                                                false,
                                                !virDomainNetTypeSharesHostView(net)));
-            ignore_value(virDomainNetBandwidthUpdate(net,
-                                                     net->bandwidth));
+            if (net->bandwidth) {
+                ignore_value(virDomainNetBandwidthUpdate(net,
+                                                         net->bandwidth));
+            }
             goto endjob;
         }