qemu: Validate available slot count for memory devices

While qemu would reject the configuration we can check whether it makes
sense to plug the device upfront.

src/qemu/qemu_command.c

@@ -8822,6 +8822,14 @@ qemuBuildCommandLine(virConnectPtr conn,
 
         /* memory hotplug requires NUMA to be enabled - we already checked
          * that memory devices are present only when NUMA is */
+
+        if (def->nmems > def->mem.memory_slots) {
+            virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                           _("memory device count '%zu' exceeds slots count '%u'"),
+                           def->nmems, def->mem.memory_slots);
+            goto error;
+        }
+
         for (i = 0; i < def->nmems; i++) {
             char *backStr;
             char *dimmStr;

src/qemu/qemu_driver.c

@@ -8489,6 +8489,12 @@ qemuDomainAttachDeviceConfig(virQEMUCapsPtr qemuCaps,
         break;
 
     case VIR_DOMAIN_DEVICE_MEMORY:
+        if (vmdef->nmems == vmdef->mem.memory_slots) {
+            virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                           _("no free memory device slot available"));
+            return -1;
+        }
+
         if (virDomainMemoryInsert(vmdef, dev->data.memory) < 0)
             return -1;
         dev->data.memory = NULL;

src/qemu/qemu_hotplug.c

@@ -1730,6 +1730,12 @@ qemuDomainAttachMemory(virQEMUDriverPtr driver,
     int id;
     int ret = -1;
 
+    if (vm->def->nmems == vm->def->mem.memory_slots) {
+        virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                       _("no free memory device slot available"));
+        goto cleanup;
+    }
+
     if (virAsprintf(&mem->info.alias, "dimm%zu", vm->def->nmems) < 0)
         goto cleanup;