conf: report an error message for non-existing USB hubs

If any of the devices referenced a USB hub that does not exist,
defining the domain would either fail with:
error: An error occurred, but the cause is unknown
(if only the last hub in the path is missing)
or crash.

Return a proper error instead of crashing.

https://bugzilla.redhat.com/show_bug.cgi?id=1367130

src/conf/domain_addr.c

@@ -1539,6 +1539,14 @@ virDomainUSBAddressFindPort(virDomainUSBAddressSetPtr addrs,
             return NULL;
         }
         hub = hub->ports[portIdx];
+        if (!hub) {
+            virReportError(VIR_ERR_XML_ERROR,
+                           _("there is no hub at port %u in USB address bus: %u port: %s"),
+                           info->addr.usb.port[i],
+                           info->addr.usb.bus,
+                           portStr);
+            return NULL;
+        }
     }
 
     *targetIdx = info->addr.usb.port[lastIdx] - 1;

tests/qemuxml2argvdata/qemuxml2argv-usb-hub-nonexistent.xml

@@ -0,0 +1,19 @@
+<domain type='qemu'>
+  <name>QEMUGuest1</name>
+  <uuid>c7a5fdbd-edaf-9455-926a-d65c16db1809</uuid>
+  <memory unit='KiB'>219136</memory>
+  <currentMemory unit='KiB'>219136</currentMemory>
+  <vcpu placement='static'>1</vcpu>
+  <os>
+    <type arch='i686' machine='pc'>hvm</type>
+    <boot dev='hd'/>
+  </os>
+  <devices>
+    <emulator>/usr/bin/qemu</emulator>
+    <controller type='usb' index='0'/>
+    <memballoon model='virtio'/>
+    <input type='mouse' bus='usb'>
+      <address type='usb' bus='0' port='1.2.3'/>
+    </input>
+  </devices>
+</domain>

tests/qemuxml2argvtest.c

@@ -1204,6 +1204,9 @@ mymain(void)
     DO_TEST_PARSE_ERROR("usb-hub-conflict",
             QEMU_CAPS_CHARDEV, QEMU_CAPS_USB_HUB,
             QEMU_CAPS_NODEFCONFIG);
+    DO_TEST_PARSE_ERROR("usb-hub-nonexistent",
+            QEMU_CAPS_CHARDEV, QEMU_CAPS_USB_HUB,
+            QEMU_CAPS_NODEFCONFIG);
     DO_TEST("usb-port-missing",
             QEMU_CAPS_CHARDEV, QEMU_CAPS_USB_HUB,
             QEMU_CAPS_NODEFCONFIG);