Fix find_disk return value issue

Previously due to both ata-*, scsi-* were parsed as /dev/sda. so
when executing "find_disk ata ALL", it will return "/dev/sda,/dev/sda",
actually we want it return "/dev/sda".
Now we make the "for loop" a bit more strict.

Change-Id: Ia5cfac7149b00f54e68a83f64cf777f071825697
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=956568
Signed-off-by: hadong <hadong0720@gmail.com>