be robust to malformed `showvminfo` output

[mxw]

recent versions of mac os x virtualbos seem to include a sort of section header for recording screen properties, which causes vm creation to fatal because it's not in the expected k=v format. let's just skip over these lines.

fixes #31

[mattoxb]

I wonder though if it would be just slightly more efficient to do something like

parts = line.split("=", 1)
if len(parts) < 2:
  continue
(k,v) = parts