desc_struct integration

Hi,

this is a series of patches that unify the struct desc_struct and friends across x86_64 and i386. As usual, it provides paravirt capabilities as a side-effect for x86_64.

I consider the main goal, namely, of unifying the desc_struct, an ongoing effort, being this the beginning. A lot of old code has to be touched to accomplish that.

I don't consider this patch ready for inclusion. Basically, the main reason is that I change the signatures of write_idt_entry(), write_gdt_entry(), and write_ldt_entry(). This is needed to account for the differences between the two architectures. (For example, gate descriptors in x86_64 are 16-byte long and can't be represented by low and high entries). As my patch series were 64-bit only, I hadn't come across the problem before.

I think this interface is sane, but I'd like to hear opinions. Specially from Jeremy and Zach, since it would touch xen and vmi code. The later, by the way, is _not_ included in this series. Being the interface acked, we still have to write it.

Thanks in advance for the review in opinions to come.