Interacting with NetBSD serial console boot blocks no longer works

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1743191
Reported by 'Andreas Gustafsson' on 2018-01-14 :

The NetBSD boot blocks display a menu allowing the user to make a
selection using the keyboard.  For example, when booting a NetBSD
installation CD-ROM, the menu looks like this:

         1. Install NetBSD
         2. Install NetBSD (no ACPI)
         3. Install NetBSD (no ACPI, no SMP)
         4. Drop to boot prompt

    Choose an option; RETURN for default; SPACE to stop countdown.
    Option 1 will be chosen in 30 seconds.

When booting NetBSD in a recent qemu using an emulated serial console,
making this menu selection no longer works: when you type the selected
number, the keyboard input is ignored, and the 30-second countdown
continues.  In older versions of qemu, it works.

To reproduce the problem, run:

   wget
http://ftp.netbsd.org/pub/NetBSD/NetBSD-7.1.1/amd64/installation/cdrom/boot-
com.iso
   qemu-system-x86_64 -nographic -cdrom boot-com.iso

During the 30-second countdown, press 4

Expected behavior: The countdown stops and you get a ">" prompt

Incorrect behavior: The countdown continues

There may also be some corruption of the terminal output; for example,
"Option 1 will be chosen in 30 seconds" may be displayed as "Option 1
will be chosen in p0 seconds".

Using bisection, I have determined that the problem appeared with qemu
commit 083fab0290f2c40d3d04f7f22eed9c8f2d5b6787, in which seabios was
updated to 1.11 prerelease, and the problem is still there as of
commit 7398166ddf7c6dbbc9cae6ac69bb2feda14b40ac.  The host operating
system used for the tests was Debian 9 x86_64.

Credit for discovering this bug goes to Paul Goyette.