qemu-vm: Add new option writableStoreUseTmpfs.

This is to use the VMs own disk image instead of a tmpfs in order to
avoid eating more memory. Of course, by default we still use the tmpfs
in order to not break existing VM tests.

I personally don't like the coding style of the option definition, but
in order to stay consistent, I followed the overall style in this file.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>