vnc: check fd before calling qemu_set_fd_handler2() in vnc_client_write()

Setting fd = -1 to qemu_set_fd_handler2() causes bus error at FD_SET
in main_loop_wait().

Signed-off-by: Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>