qemu: Enable PCI multi bus for S390 guests

QEMU on s390 supports PCI multibus since forever.
Signed-off-by: NYi Min Zhao <zyimin@linux.ibm.com>
Reviewed-by: NBoris Fiuczynski <fiuczy@linux.ibm.com>
Reviewed-by: NStefan Zimmermann <stzi@linux.ibm.com>
Reviewed-by: NBjoern Walk <bwalk@linux.ibm.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>

src/qemu/qemu_capabilities.c

@@ -1702,6 +1702,10 @@ bool virQEMUCapsHasPCIMultiBus(virQEMUCapsPtr qemuCaps,
         return false;
     }
 
+    /* S390 supports PCI-multibus. */
+    if (ARCH_IS_S390(def->os.arch))
+        return true;
+
     /* If ARM 'virt' supports PCI, it supports multibus.
      * No extra conditions here for simplicity.
      */