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 initalized. 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.