cmake: emu: Add VIRTIO console to QEMU CMake file

Replace the default serial device with a virtconsole if it
is configured in the devicetree. Because of the way -nographic
works, -serial none and -monitor none had to be added as well.
Tested with west build -t run and west build -t debugserver_qemu.

Signed-off-by: Jakub Klimczak <jklimczak@internships.antmicro.com>