char: move CharBackend handling in char-fe unit

Move all the frontend struct and methods to a seperate unit. This avoids
accidentally mixing backend and frontend calls, and helps with readabilty.

Make qemu_chr_replay() a macro shared by both char and char-fe.

Export qemu_chr_write(), and use a macro for qemu_chr_write_all()

(nb: yes, CharBackend is for char frontend :)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>