target-i386: Use xsave structs for ext_save_area

This doesn't introduce any change in the code, as the offsets and
struct sizes match what was present in the table. This can be
validated by the QEMU_BUILD_BUG_ON lines on target-i386/cpu.h,
which ensures the struct sizes and offsets match the existing
values in ext_save_area.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
