chardev: new qemu_chr_be_update_read_handlers()

Add a wrapper for the chr_update_read_handler().

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1505975754-21555-2-git-send-email-peterx@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>