tests: Don't use memballoon=none for minimal tests

tests: Don't use memballoon=none for minimal tests

It's redundant (these machine types don't get a memballoon added
automatically anyway), plus the test is supposed to show what
happens when a minimal configuration is fed to libvirt and
including additional elements goes against that.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>