Port qemu-kvm's signalfd compat code.

commit 5a7fdd0abd7cd24dac205317a4195446ab8748b5
Author: Anthony Liguori <aliguori@us.ibm.com>
Date:   Wed May 7 11:55:47 2008 -0500

    Use signalfd() in io-thread

    This patch reworks the IO thread to use signalfd() instead of sigtimedwait()
    This will eliminate the need to use SIGIO everywhere.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>