qemu_chr_fe_write() is similar to qemu_chr_write_all(): the later write
all with a chardev backend.

Make qemu_chr_write() and qemu_chr_fe_write_buffer() take an 'all'
argument. If false, handle 'partial' write the way qemu_chr_fe_write()
use to, and call qemu_chr_write() from qemu_chr_fe_write().

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