Check for missing line in files recursively and add it

Hi, I need a shell, python, or perl script i can run in cron at 1 min interval temporarily on CentOS6 that does the following. It searches /home/xen recursively looking for the following line in all .cfg files it finds in all subdirectories. This line exactly: device_model_version = 'qemu-xen-traditional' Only if it's missing it adds it to the end of the file Example directory structure showing multiple files it would need to iterate through in different subdirectories of /home/xen [root@xenhost1 xen]# pwd /home/xen [root@xenhost1 xen]# ls -Rl .: total 8 drwxr-xr-x 3 root root 4096 Feb 19 16:15 vm101 drwxr-xr-x 3 root root 4096 Feb 18 15:58 vm102 ./vm101: total 8 drwxr-xr-x 2 root root 4096 Dec 15 14:50 mnt -rw-r--r-- 1 root root 755 Feb 18 15:58 [login to view URL] ./vm101/mnt: total 0 ./vm102: total 8 drwxr-xr-x 2 root solusvm 4096 Feb 12 16:55 mnt -rw-r--r-- 1 solusvm solusvm 755 Feb 18 15:58 [login to view URL] ./vm102/mnt: total 0 [root@xenhost1 xen]#