util/bufferiszero: assign length_to_accel value for each accelerator case

Because in unit test, init_accel() will be called several times, each with
different accelerator type.

Signed-off-by: Robert Hoo <robert.hu@linux.intel.com>
Message-Id: <1585119021-46593-1-git-send-email-robert.hu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
1 file changed