The iomap_base of TaskStateSegment shouldn't be initialized to zero

[josephlr]

Right now TaskStateSegment::new() initializes iomap_base to 0. This is wrong if we want a "new" TSS to represent one without an IO Map, as this causes the TSS itself to be interpreted as an IO map.

Initializing it to size_of::<TaskStateSegment>(), seems more correct. When creating a segment with tss_segment, this would make it so there is actually not an IO map.

Discovered when reviewing #194

[Restioson]

Should I just address this in #194?

[josephlr]

It will depend on exactly what interface we end up with, but I think a separate PR will be better.