It is not possible to use virtio-ioeventfd when building without an I/O
thread.  We rely on a signal to kick us out of vcpu execution.  Timers
and AIO use SIGALRM and SIGUSR2 respectively.  Unfortunately eventfd
does not support O_ASYNC (SIGIO) so eventfd cannot be used in a signal
driven manner.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>