Do not allow AIO to be inited multiple times

This prevents two signalfd() threads from being spawned.  This problem was
originally spotted by Blue Swirl.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>



git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5289 c046a42c-6fe2-441c-8c8c-71466251a162
