utils: Fix UnescapeAndSplit parsing bug

If a value passed to UnescapeAndSplit ended with a backslash an
exception would be raised:

$ gnt-instance modify -H mem=x\\ inst1.example.com
[…]
    e2 = slist.pop(0)
IndexError: pop from empty list

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>