utils_net: fix dhcp client selection logic and restart_cmd assignment

1. fix RHEL >= 10 dhcp logic wrongly applied to guest, limit it to
host side only.
2. fix restart_cmd assignment

Signed-off-by: Wenli Quan <wquan@redhat.com>

Author: quanwenli

virttest/utils_net.py

@@ -1718,10 +1718,13 @@ def get_dhcp_client(session):
     :return: tuple of dhcp command and its release argument, raises TestError if none found
     """
     dhcp_clients = [("dhclient", "-r"), ("dhcpcd", "-k")]
-    if distro.detect().name == "rhel" and int(distro.detect().version) >= 10:
-        dhcp_clients = [dhcp_clients[1]]
-    else:
-        dhcp_clients = [dhcp_clients[0]]
+
+    if session is None:
+        if distro.detect().name == "rhel" and int(distro.detect().version) >= 10:
+            dhcp_clients = [dhcp_clients[1]]
+        else:
+            dhcp_clients = [dhcp_clients[0]]
+
     for cmd, release_flag in dhcp_clients:
         status, _ = utils_misc.cmd_status_output(
             "which %s" % cmd, shell=True, ignore_status=True, session=session
@@ -1758,7 +1761,7 @@ def restart_guest_network(
             else:
                 restart_cmd += "%s %s" % (dhcp_cmd, nic_ifname)
         else:
-            restart_cmd += "%s %s; " % (dhcp_cmd, release_flag)
+            restart_cmd = "%s %s; " % (dhcp_cmd, release_flag)
             if ip_version == "ipv6":
                 restart_cmd += "%s -6" % dhcp_cmd
             else: