hw/char: QOM'ify etraxfs_ser.c

* Drop the old SysBus init function and use instance_init
* Call qemu_chr_add_handlers in the realize callback
* Use qdev chardev prop instead of qemu_char_get_next_serial
* Add etraxfs_ser_create function to create etraxfs serial device

Signed-off-by: xiaoqiang zhao <zxq_yx_007@163.com>
Message-Id: <1464158344-12266-3-git-send-email-zxq_yx_007@163.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>