qemu-nbd: Switch to qemu_set_fd_handler

Achieved by:

- Remembering the server fd with a global variable, in order to access
  it from nbd_client_closed.

- Checking nbd_can_accept() and updating server_fd handler whenever
  client connects or disconnects.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1432032670-15124-3-git-send-email-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
