Display any qemu error output if it gets displayed

[ramereth]

Right now its really difficult to know what goes wrong with the qemu provider. Most of the time its something with qemu itself thus it would be great to see the stderr of qemu on a run if it hits an error.

[tdhite]

I agree, now that you mention it. I'll get stderr out to the console log (as opposed to forcing looks into packer.log).