用虚拟机安装红旗服务器4.1进入不了图形模式

我在虚拟机上安装红旗服务器4.1成功之后自动是文本模式。用root账号登陆后运行startx后就出现：Fatal server error:
no screens found
When reporting a problem related to a sever crash, please send the full server output, not just the last messages.
This can be found in the log file"/var/log/XFree86.0.log".
Please report problems to XFree86@XFree86.org.
.......
怎么也进入不了图形模式。请问是哪个地方出了问题？