blockdev: Disentangle BlockDriverState and DriveInfo creation

blockdev_init() mixes up BlockDriverState and DriveInfo initialization
Finish the BlockDriverState job before starting to mess with
DriveInfo.  Easier on the eyes.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Benoît Canet <benoit.canet@nodalink.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>