util/bufferiszero: improve avx2 accelerator

util/bufferiszero: improve avx2 accelerator

By increasing avx2 length_to_accel to 128, we can simplify its logic and reduce a
branch.

The authorship of this patch actually belongs to Richard Henderson
<richard.henderson@linaro.org>, I just fixed a boundary case on his
original patch.

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Robert Hoo <robert.hu@linux.intel.com>
Message-Id: <1585119021-46593-2-git-send-email-robert.hu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>