spice: fix spice_chr_add_watch() pre-condition

Since e02bc6de30c44fd668dc0d6e1cd1804f2eed3ed3, add_watch() is called
with G_IO_HUP. Even if spice-qemu-char ignores this flag, the
precondition must be changed.

https://bugzilla.redhat.com/show_bug.cgi?id=1128992

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
