tests: qemublock: Add test cases for 'aio' options of 'file' storage

tests: qemublock: Add test cases for 'aio' options of 'file' storage

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: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>