tests: qemuxml2argv: Use proper replacement for 'pc-i440fx-wily' machine

tests: qemuxml2argv: Use proper replacement for 'pc-i440fx-wily' machine

This machine type comes from downstream ubuntu 15.10. Replace it with a
somewhat equivalent qemu-2.3 machine type as we do have test data for
that.

The change allows the CPU code to pick a proper default CPU in the
'-latest' cases.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>