Add one case of virtual network

1.Reboot guest multiple times with multiple NICs
Signed-off-by: NLei Yang <leiyang@redhat.com>

qemu/tests/cfg/multi_nics_reboot.cfg

+- multi_nics_reboot:
+    virt_test_type = qemu
+    type = boot
+    serial_login = no
+    reboot_method = shell
+    nics = nic1 nic2 nic3 nic4
+    restart_vm = yes
+    kill_vm_on_error = yes
+    login_timeout = 240
+    reboot_count = 10