qemu: Use virtconsole serial console instead of regular serial console

qemu: Use virtconsole serial console instead of regular serial console

With --qemu-headless, qemu stops working when latest systemd is not
available because smbios provided kernel cmdline arguments are ignored.
This includes "console=ttyS0" meaning we don't get any output on the
serial console when booting in qemu.

As a workaround, let's use qemu's virtconsole instead. This means the
serial console will be available in the VM as /dev/hvc0, on which
systemd will automatically spawn a serial getty. This means we'll
eventually get a login prompt, even if latest systemd is not used.

One caveat is that EDK2 doesn't seem to support virtio-serial with
virtconsole. To get around this limitation, we keep a regular serial
console around as well, which can be used by the bootloader to log
its output.