qemu: Auto add pci-root for s390/s390x guests

The pci-root depends on zpci capability. So autogenerate pci-root if
zpci exists.
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_domain.c

@@ -3290,6 +3290,7 @@ qemuDomainDefAddDefaultDevices(virDomainDefPtr def,
     case VIR_ARCH_S390X:
         addDefaultUSB = false;
         addPanicDevice = true;
+        addPCIRoot = virQEMUCapsGet(qemuCaps, QEMU_CAPS_DEVICE_ZPCI);
         break;
 
     case VIR_ARCH_SPARC: