blockdev: Make drive_add() take explicit type, index parameters
Before, type & index were hidden in printf-like fmt, ... parameters,
which get expanded into an option string. Rather inconvenient for
uses later in this series.
New IF_DEFAULT to ask for the machine's default interface. Before,
that was done by having no option "if" in the option string.
Signed-off-by: Markus Armbruster <armbru@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>