viorng_in_use: correct re pattern

"grep [Rs,Ds]" is not to get "Rs" OR "Ds".
Change it to "grep 'Rs\|Ds'".
Signed-off-by: Nybduan <yduan@redhat.com>

qemu/tests/cfg/viorng_in_use.cfg

@@ -28,7 +28,7 @@
         target_process = rngd
         driver_name = virtio_rng
         rng_data_rex = "\w+"
-        list_cmd = "ps aux | grep rngd | grep [Rs,Ds]"
+        list_cmd = "ps aux | grep rngd | grep 'Rs\|Ds'"
         check_rngd_service = "systemctl status rngd"
         start_rngd_service = "systemctl start rngd"
         RHEL.6: