QtIFW

Check for valid drive. Disallow e.g. C:foo\bar

Task-number: QTIFW-165

Once we add a something after the colon and before the next slash,
the path will be interpreted relative to the current directory,
thus leading to unexpected installation directory.

Change-Id: I0f32f150707d1b3ca55bd93f230142dad4f7540a
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>