qemuxml2argvtest: Modernize 'cpu-(no)fallback' test

qemuxml2argvtest: Modernize 'cpu-(no)fallback' test

Previously without modern capabilities the test was relying on a CPU
model which was not entered into a fake list of supported cpus.

With real capabilities we have to pick a CPU model which is supported by
libvirt but in some version is not supported by qemu. I've picked
EPYC-Milan, which was introduced into qemu-6.0.

This test configures a CPU which is equivalent to EPYC-Rome by disabling
features from EPYC-Milan and uses a versioned real caps test to check it
against a qemu which doesn't support EPYC-Milan.

With real capabilities though, we can also do a positive test case by
using a version whic doesh support it. I've specifically not used the
LATEST caps so that it doesn't change once capabilities are bumped.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>