lguest: switch segment-voodoo-numbers to readable symbols

When studying lguest's x86 segment descriptor code, it is not longer
necessary to have the Intel x86 architecture manual open on the page
with the segment descriptor illustration to understand the crazy
numbers assigned to both descriptor structure halves a/b.
Now the struct desc_struct's fields, like suggested by
Glauber de Oliveira Costa in 2008, are used.

Signed-off-by: Jacek Galowicz <jacek@galowicz.de>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>