qemu-capabilities: Fix query-cpu-model-expansion on s390 with older kernel

When running on s390 with a kernel that does not support cpu model checking and
with a Qemu new enough to support query-cpu-model-expansion, the gathering of qemu
capabilities will fail. Qemu responds to the query-cpu-model-expansion qmp
command with an error because the needed kernel ioct does not exist. When this
happens a guest cannot even be defined due to missing qemu capabilities data.

This patch fixes the problem by silently ignoring generic errors stemming from
calls to query-cpu-model-expansion.
Reported-by: NFarhan Ali <alifm@linux.vnet.ibm.com>
Signed-off-by: NCollin L. Walling <walling@linux.vnet.ibm.com>
Signed-off-by: NJason J. Herne <jjherne@linux.vnet.ibm.com>

src/qemu/qemu_monitor_json.c

@@ -5032,6 +5032,15 @@ qemuMonitorJSONGetCPUModelExpansion(qemuMonitorPtr mon,
     if (qemuMonitorJSONCommand(mon, cmd, &reply) < 0)
         goto cleanup;
 
+    /* Even though query-cpu-model-expansion is advertised by query-commands it
+     * may just return GenericError if it is not implemented for the requested
+     * guest architecture or it is not supported in the host environment.
+     */
+    if (qemuMonitorJSONHasError(reply, "GenericError")) {
+        ret = 0;
+        goto cleanup;
+    }
+
     if (qemuMonitorJSONCheckError(cmd, reply) < 0)
         goto cleanup;