CVE-2018-3639 (x86)

This patch removes magic number with enum cpuid_leafs.
Signed-off-by: Huaitong Han <huaitong.han@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(backported from commit e0b18ef7)
[juergh:- Context adjustments.]
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>