quick fix for varnishstat memory problem

varnishstat Memory graph broken as it output the value in this form:
```multigraph varnish_memory
SMA.s0.g_bytes.value 1203772840
SMA.s0.g_space.value 473948760```

Which confuse munin's value parsing protocol.

I did a quick fix for this, but I believe it's far from perfect, simply hacky.
I find there's the autoFixname param in `MuninGraph`, but it's not working even if set, for the values got from `pysysinfo/vanish` are with dots, fixed names can no longer get value from the stat dict. 

I suggest to keep the fixName mechanism as simple as possible:  keep internal names so that they can be coordinated with each module, only fix them when output to munin protocol.

I'm not sure if this would break other plugins so I left the work for you, and it's ok if no one want's to touch that and keep this hacky patch ...  :)