Test that the 'aio' option is applied correctly for the 'file' protocol
backend and across the backing chain.

The top level disk image would generate the following '-drive' cmdline:

file-backing_basic-aio_threads:
-drive file=/var/lib/libvirt/images/a,format=qcow,if=none,id=drive-dummy,aio=threads
-device virtio-blk-pci,scsi=off,drive=drive-dummy,id=dummy

file-raw-aio_native:
-drive file=/path/to/i.img,format=raw,if=none,id=drive-dummy,cache=none,aio=native
-device virtio-blk-pci,scsi=off,drive=drive-dummy,id=dummy,write-cache=on
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>