libguestfs: Add 2 funcs for virt-command

1. Get virt-sysprep operation list
2. Check if option is supported by virt-command

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
thanks for pointing it out. I just got a simple way:
```
opt = " " + opt + " "
return (result.stdout.count(opt) != 0)
```
will update soon.