qemuxml2argvtest: Drop dependency between testInfoArgName and virQEMUCapsFlags enums

Introduced in fdf6c89ee7b, this dependency looks weird. It was
needed because of the way that while() loop was written - it
fetches next argument in every iteration. Therefore, our only
option was for ARG_END to have the same value as QEMU_CAPS_LAST.
This also meant that QEMU_CAPS_* could have been only at the end
of the __VA_ARGS__.

This commit reworks the while() loop and removes the dependency.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>