utils_net: get default gateway for specific interface

Add optional parameter in order to retrieve the default gateway
for a specific interface.

On a system with two interfaces, the routes might look like

```
default via 10.172.161.254 dev enc1 proto dhcp src 10.172.160.70 metric 100
default via 192.168.126.1 dev enc2 proto dhcp src 192.168.126.48 metric
102
```

Filtering only by default will return two ips

```
10.172.161.254
192.168.126.1
```

The change allows us to select the interface and thus retrieve only

```
192.168.126.1
```

Signed-off-by: Sebastian Mitterle <[email protected]>

virttest/utils_net.py

@@ -3893,14 +3893,15 @@ def get_host_iface():
 
 
 def get_default_gateway(iface_name=False, session=None, ip_ver='ipv4',
-                        force_dhcp=False):
+                        force_dhcp=False, target_iface=None):
     """
     Get the Default Gateway or Interface of host or guest.
 
     :param iface_name: Whether default interface (True), or default gateway
                         (False) is returned, defaults to False
     :param session: shell/console session if any, defaults to None
     :param ip_ver: ip version, defaults to 'ipv4'
+    :param target_iface: if given, get default gateway only for this device
     :return: default gateway of target iface
     """
     if ip_ver == 'ipv4':
@@ -3911,10 +3912,14 @@ def get_default_gateway(iface_name=False, session=None, ip_ver='ipv4',
         raise ValueError(f'Unrecognized IP version {ip_ver}')
     if force_dhcp:
         ip_cmd = ip_cmd + '|grep dhcp'
+    if target_iface:
+        regex = "default.*%s" % target_iface
+    else:
+        regex = "default"
     if iface_name:
-        cmd = "%s | awk '/default/ { print $5 }'" % ip_cmd
+        cmd = "%s | awk '/%s/ { print $5 }'" % (ip_cmd, regex)
     else:
-        cmd = "%s | awk '/default/ { print $3 }'" % ip_cmd
+        cmd = "%s | awk '/%s/ { print $3 }'" % (ip_cmd, regex)
     try:
         _, output = utils_misc.cmd_status_output(cmd, shell=True, session=session)
         if session: