s390x/cpumodel: fix feature groups and breakage of MSA8

Since commit 46a99c9f ("s390x/cpumodel: model PTFF subfunctions
for Multiple-epoch facility") -cpu help no longer shows the MSA8
feature group. Turns out that we forgot to add the new MEPOCH_PTFF
group enum.

Fixes: 46a99c9f ("s390x/cpumodel: model PTFF subfunctions for Multiple-epoch facility")
Reviewed-by: NDavid Hildenbrand <david@redhat.com>
Signed-off-by: NChristian Borntraeger <borntraeger@de.ibm.com>

target/s390x/cpu_features.h

@@ -78,6 +78,7 @@ typedef enum {
     S390_FEAT_GROUP_MSA_EXT_6,
     S390_FEAT_GROUP_MSA_EXT_7,
     S390_FEAT_GROUP_MSA_EXT_8,
+    S390_FEAT_GROUP_MULTIPLE_EPOCH_PTFF,
     S390_FEAT_GROUP_MAX,
 } S390FeatGroup;