block: simplify default_drive Markus Armbruster pointed out that there is only one caller to default_drive with IF_DEFAULT as a type. Lets get rid of the block_default_type parameter and adopt the caller to do the right thing (asking the machine struct). Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com> Reviewed-by: Markus Armbruster <armbru@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>