s390x/ccw: create s390 phb for compat reasons as well

d32bd032d8 ("s390x/ccw: create s390 phb conditionally") made
registering the s390 pci host bridge conditional on presense
of the zpci facility bit. Sadly, that breaks migration from
machines that did not use the cpu model (2.7 and previous).

Create the s390 phb for pre-cpu model machines as well: We can
tweak s390_has_feat() to always indicate the zpci facility bit
when no cpu model is available (on 2.7 and previous compat machines).

Fixes: d32bd032d8 ("s390x/ccw: create s390 phb conditionally")
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

target/s390x/cpu_models.c

@@ -196,6 +196,9 @@ bool s390_has_feat(S390Feat feat)
             }
         }
 #endif
+        if (feat == S390_FEAT_ZPCI) {
+            return true;
+        }
         return 0;
     }
     return test_bit(feat, cpu->model->features);