[libvirt] Libvirt xl to xml converter only picks up first occurrence of an option

[Wei Liu]

Hi Jim

I discovered that libvirt's native config file to xml converter for
libxl only pick up the first occurrence of an option.

For example in a xl cfg file:

extra = "abc"
...
extra = "def"

xl will pick up "def" because that shows up later and takes precedence,
while the converter picks up "abc".

I think this is a bug in the converter and should be fixed.

Thanks
Wei.