qemu.tests: skip generate serivce file if it exist

udev in latest rhel linux can generate serivce file for new
vport and vconsole, skip generate if the file exist in guest
os to avoid unexpect exception.
Signed-off-by: NXu Tian <xutian@redhat.com>

qemu/tests/cfg/virtio_port_login.cfg

@@ -15,7 +15,7 @@
             RHEL.6:
                 pre_cmd += "sed -i '/ACTIVE_CONSOLES/c ACTIVE_CONSOLES="/dev/tty[1-6] /dev/vport0p1"' /etc/sysconfig/init;"
             RHEL.7:
-                pre_cmd += "ln -s /usr/lib/systemd/system/serial-getty@.service /etc/systemd/system/serial-getty@vport0p1.service;"
+                pre_cmd += "systemctl |grep vport0 || ln -s /usr/lib/systemd/system/serial-getty@.service /etc/systemd/system/serial-getty@vport0p1.service;"
                 pre_cmd += "systemctl enable serial-getty@vport0p1.service"
         - virtio_console:
             login_console = vc1
@@ -23,5 +23,5 @@
             RHEL.6:
                 pre_cmd += "sed -i '/ACTIVE_CONSOLES/c ACTIVE_CONSOLES="/dev/tty[1-6] /dev/hvc0"' /etc/sysconfig/init;"
             RHEL.7:
-                pre_cmd += "ln -s /usr/lib/systemd/system/serial-getty@.service /etc/systemd/system/serial-getty@hvc0.service;"
+                pre_cmd += "systemctl | grep hvc0 || ln -s /usr/lib/systemd/system/serial-getty@.service /etc/systemd/system/serial-getty@hvc0.service;"
                 pre_cmd += "systemctl enable serial-getty@hvc0.service"