tools/insn-fuzz: Correct hook prototypes, and assert() appropriate segments

tools/insn-fuzz: Correct hook prototypes, and assert() appropriate segments

The correct prototypes for the hooks are to use enum x86_segment rather than
unsigned int.  It is implementation specific as to whether this compiles.

assert() that the emulator never passes an inappropriate segment.  The only
hook which may legitimately be passed x86_seg_none is invlpg().

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