ga_channel_open() was using open flag O_ASYNC for SIGIO-driven I/O.
This breaks on illumos, so fall back to POSIX I_SETSIG ioctl (SIGPOLL).

Signed-off-by: Lee Essen <lee.essen@nowonline.co.uk>
Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>