cmake: qemu: Use chardev for console

The current QEMU console configuration directly connects the console
serial port to the backend using '-serial' option.

This is less than ideal because only single console instance can be
connected to a backend and aggregation of multiple console outputs is
not possible (e.g. multiple console serial ports and semihosting
console to single console backend).

In order to solve this problem, single multiplexed chardev console
backend is declared and all consoles are connected to it.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>