How to redirect QEMU -serial output to both a file and the terminal or a port?

Recently, I ran into the exactly same problem and found a solution:

According to QEMU 3.1.0 documentation, you can use a chardev with options stdio and logfile and redirect your serial into it.

qemu-system-x86_64 -chardev stdio,id=char0,logfile=serial.log,signal=off \
  -serial chardev:char0

Moreover, if you use mux=on option within chardev, you can redirect a monitor (mon), another serial or other interfaces supporting chardev backend into it simultaneously:

qemu-system-x86_64 -chardev stdio,id=char0,mux=on,logfile=serial.log,signal=off \
  -serial chardev:char0 -mon chardev=char0

See the example in the doc. link above.