[S390] cio: Use add_uevent_var.

Convert ccw_uevent to use add_uevent_var and adapt snprint_alias.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>