blockdev: New drive_get_next(), replacing qdev_init_bdrv()

qdev_init_bdrv() doesn't belong into qdev.c; it's about drives, not
qdevs.  Rename to drive_get_next, move to blockdev.c, drop the bogus
DeviceState argument, and return DriveInfo instead of
BlockDriverState.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>