fix(common): Exclude default systemd filesystem targets.

We don't need the default root filesystem fsck and remount targets
provided by systemd since root is read only. The only default one what
was included in this way was tmp.mount but that is now covered by
a dependency in the coreos-init package.
fix(image_to_vm): Cleanup qemu script, document -virtfs
Merge pull request #84 from marineam/virtfs

Virtfs bits