This was needed when qemu-nbd was using qemu_set_fd_handler2.  It is
not needed anymore now that nbd_update_server_fd_handler is called
whenever nbd_can_accept() can change from false to true.
nbd_update_server_fd_handler will call qemu_set_fd_handler(),
which will call qemu_notify_event().

Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>