i386: hvf: Fix overrun of _decode_tbl1

Single opcode instructions in ff group were incorrectly processed
because an overrun of _decode_tbl1[0xff] resulted in access of
_decode_tbl2[0x0]. Thus, decode_sldtgroup was called instead of
decode_ffgroup:
  7d71: decode_sldtgroup: 1
  Unimplemented handler (7d71) for 108 (ff 0)

While at it correct maximum length for _decode_tbl2 and _decode_tbl3.

Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

target/i386/hvf/x86_decode.c

@@ -454,9 +454,9 @@ struct decode_x87_tbl {
 struct decode_tbl invl_inst = {0x0, 0, 0, false, NULL, NULL, NULL, NULL,
                                decode_invalid};
 
-struct decode_tbl _decode_tbl1[255];
-struct decode_tbl _decode_tbl2[255];
-struct decode_x87_tbl _decode_tbl3[255];
+struct decode_tbl _decode_tbl1[256];
+struct decode_tbl _decode_tbl2[256];
+struct decode_x87_tbl _decode_tbl3[256];
 
 static void decode_x87_ins(CPUX86State *env, struct x86_decode *decode)
 {