qemu: report an error if memballoon has wrong address type

qemuBuildMemballoonDevStr returns NULL if memballoon doesn't have
the right address type, but it doesn't report an error, leading to:
error: An error occurred, but the cause is unknown

Report a helpful error message instead, e.g.:
error: XML error: memballoon unsupported with address type 'usb'

src/qemu/qemu_command.c

@@ -4175,6 +4175,9 @@ qemuBuildMemballoonDevStr(virDomainMemballoonDefPtr dev,
             virBufferAddLit(&buf, "virtio-balloon-ccw");
             break;
         default:
+            virReportError(VIR_ERR_XML_ERROR,
+                           _("memballoon unsupported with address type '%s'"),
+                           virDomainDeviceAddressTypeToString(dev->info.type));
             goto error;
     }