ui/console-vc: extract vt100_keysym() from qemu_text_console_handle_keysym()

Move the keysym handling logic out of qemu_text_console_handle_keysym()
into a new vt100_keysym() helper that operates on QemuVT100 directly,
continuing the effort to decouple the VT100 layer from the console layer.

The echo path is updated to call vt100_input() instead of
qemu_chr_write(), since the function no longer has direct access
to the chardev.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>