tests: qemuxml2argv: Use 64 bit qemu binary and 1.5 machine type in 'disk-cache'...

tests: qemuxml2argv: Use 64 bit qemu binary and 1.5 machine type in 'disk-cache' case

The data is tested against the latest qemu binaries so we should use the
proper architecture. Also the test is used against data from qemu 1.5.3
and thus we should use a machine type that qemu supported.

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