vlan: update error message pattern for new guest

and replace "iptables -F" with stopping "firewalld.service"

1. Developer added extack messages for vlan link create in kernel
> 4.18, so update corresponding txt pattern in test case.

2. "iptables -F" deletes all the filter rules on RHEL7, but does
not work on RHEL8, "firewalld" uses "nftables" as its default
backend on RHEL8. so try to stop filter rules with
"systemctl stop firewalld" on RHEL7 and RHEL8.
Signed-off-by: NSitong Liu <siliu@redhat.com>

generic/tests/cfg/vlan.cfg

@@ -13,7 +13,9 @@
     kill_vm_vm2 = yes
     kill_vm_gracefully_vm2 = no
     cmd_type = ip
-    msg_pattern = 'RTNETLINK answers: Numerical result out of range'
+    msg_pattern = '8021q: Invalid VLAN id'
+    RHEL.7, RHEL.6:
+        msg_pattern = 'RTNETLINK answers: Numerical result out of range'
     Windows:
         # two windows VM, need more time to boot and login
         login_timeout = 720

generic/tests/vlan.py

@@ -202,7 +202,8 @@ def run(test, params, env):
         digest_origin.append(re.findall(r'(\w+)', md5sum_output)[0])
 
         # stop firewall in vm
-        session.cmd_output_safe("iptables -F; service iptables stop; true")
+        stop_firewall_cmd = "systemctl stop firewalld||service firewalld stop"
+        session.cmd_output_safe(stop_firewall_cmd)
         error_context.context("Load 8021q module in guest %s" % vm.name,
                               logging.info)
         session.cmd_output_safe("modprobe 8021q")