type: enable zero value for BackendStore

Problem: There is no way one can identify if a BackendStore is
uninitialized (a.k.a has zero value) or initialized. It always will
return the type `Btrfs`. Also the `String()` method never will return the
string `<INVALID>`

Solution: Increase the enum just one by one. This will make
uninitialized values of BackendStore as 0. One can then check easily
with:

	store := BackendStore

	if store == 0 {
		// uninitialized
	}

Also the `String()` methods is going to return the `<INVALID>` string.