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>