x86: Switch "struct desc_struct" to being seg_desc_t

x86: Switch "struct desc_struct" to being seg_desc_t

The struct suffix is redundant in the name, and a future change will want to
turn it into a union, rather than a structure.  As this represents a segment
descriptor, give it an appropriate typedef.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>