vm/windows: Fix perms of the Samba shares.

We now map all guest accounts to the root user, because in the end the
permissions of the current user boil down to the build user of the Nix
builder of the host. That way it's not possible to gain more permissions
at all and just makes the VM communication a lot easier.

However, setting "writable" to yes instead of "read only" to no doesn't
change anything here, I just found it to be clearer.

Also, we now no longer need to have a "nobody" user.

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