dev: Make the IDE controller handle NULL simobject pointers.

dev: Make the IDE controller handle NULL simobject pointers.

Only panic if there are disks which would actually be connected to it beyond
its limit. Also skip past disks which are set to NULL. This is useful since
it lets you set up disks on different ports of the controller instead of
filling them contiguously.

Change-Id: I92f1316d3ad6931e25bfffeb34fb2603c0b95ce7
Reviewed-on: https://gem5-review.googlesource.com/4848
Maintainer: Gabe Black <gabeblack@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>