char-udp: flush as much buffer as possible

Instead of flushing the buffer byte by byte, call qemu_chr_be_write()
with as much byte possible accepted by the front-end.

Factor out buffer flushing in a common function udp_chr_flush_buffer().

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