conf: don't crash on a tpm device with no backends

Print an error instead of crashing when a TPM device without
a backend is specified.

Add a test for tpm device with no backend, which should fail
with a parse error.

https://bugzilla.redhat.com/show_bug.cgi?id=961252

src/conf/domain_conf.c

@@ -6988,6 +6988,12 @@ virDomainTPMDefParseXML(const xmlNodePtr node,
         goto error;
     }
 
+    if (nbackends == 0) {
+        virReportError(VIR_ERR_XML_ERROR, "%s",
+                       _("missing TPM device backend"));
+        goto error;
+    }
+
     if (!(backend = virXMLPropString(backends[0], "type"))) {
         virReportError(VIR_ERR_XML_ERROR, "%s",
                        _("missing TPM device backend type"));

tests/qemuxml2argvdata/qemuxml2argv-tpm-no-backend-invalid.xml

+<domain type='qemu'>
+  <name>TPM-VM</name>
+  <uuid>11d7cd22-da89-3094-6212-079a48a309a1</uuid>
+  <memory unit='KiB'>2097152</memory>
+  <currentMemory unit='KiB'>512288</currentMemory>
+  <vcpu placement='static'>1</vcpu>
+  <os>
+    <type arch='x86_64' machine='pc-0.12'>hvm</type>
+    <boot dev='hd'/>
+    <bootmenu enable='yes'/>
+  </os>
+  <features>
+    <acpi/>
+  </features>
+  <clock offset='utc'/>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>restart</on_reboot>
+  <on_crash>destroy</on_crash>
+  <devices>
+    <emulator>/usr/bin/qemu</emulator>
+    <controller type='usb' index='0'/>
+    <controller type='pci' index='0' model='pci-root'/>
+    <tpm model='tpm-tis'>
+    </tpm>
+    <memballoon model='virtio'/>
+  </devices>
+</domain>

tests/qemuxml2argvtest.c

@@ -970,6 +970,8 @@ mymain(void)
 
     DO_TEST("tpm-passthrough", QEMU_CAPS_DEVICE,
             QEMU_CAPS_DEVICE_TPM_PASSTHROUGH, QEMU_CAPS_DEVICE_TPM_TIS);
+    DO_TEST_PARSE_ERROR("tpm-no-backend-invalid", QEMU_CAPS_DEVICE,
+                        QEMU_CAPS_DEVICE_TPM_PASSTHROUGH, QEMU_CAPS_DEVICE_TPM_TIS);
 
     DO_TEST("pci-autoadd-addr", QEMU_CAPS_DEVICE, QEMU_CAPS_DEVICE_PCI_BRIDGE);
     DO_TEST("pci-autoadd-idx", QEMU_CAPS_DEVICE, QEMU_CAPS_DEVICE_PCI_BRIDGE);