vt82c686: avoid out-of-bounds read

superio_ioport_readb can read the 256th element of the array.
Coverity reports an out-of-bounds write in superio_ioport_writeb,
but it does not show the corresponding out-of-bounds read
because it cannot prove that it can happen.  Fix the root
cause of the problem (zhanghailang's patch instead fixes
the logic in superio_ioport_writeb).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Cc: qemu-stable@nongnu.org
