qemu.tests.single_driver_install:Update virtio serial device name

According to new policy,all RHEL host will use the latest virtio-win drivers.
And latest virtio-win serial driver use "VirtIO Serial Driver".
Signed-off-by: NYanan Fu <yfu@redhat.com>

qemu/tests/cfg/single_driver_install.cfg

@@ -62,8 +62,6 @@
         - with_vioserial:
             driver_name = vioser
             device_name = "VirtIO Serial Driver"
-            Host_RHEL.m6, Host_RHEL.m7.u0, Host_RHEL.m7.u1, Host_RHEL.m7.u2:
-                device_name = "VirtIO-Serial Driver"
             virtio_ports = "vs"
             virtio_port_type = serialport
         - with_balloon: