qmp: Expose MachineClass::default_ram_id

If a management application (like Libvirt) want's to preserve
migration ability and switch to '-machine memory-backend' it
needs to set exactly the same RAM id as QEMU would. Since the id
is machine type dependant, expose it under 'query-machines'
result. Some machine types don't have the attribute set (riscv
family for example), therefore the QMP attribute must be
optional.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Message-Id: <9384422f63fe594a54d801f9cb4539b1d2ce9b67.1590481402.git.mprivozn@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
[ehabkost: updated doc to "since 5.2"]
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

hw/core/machine-qmp-cmds.c

@@ -238,6 +238,10 @@ MachineInfoList *qmp_query_machines(Error **errp)
             info->default_cpu_type = g_strdup(mc->default_cpu_type);
             info->has_default_cpu_type = true;
         }
+        if (mc->default_ram_id) {
+            info->default_ram_id = g_strdup(mc->default_ram_id);
+            info->has_default_ram_id = true;
+        }
 
         entry = g_malloc0(sizeof(*entry));
         entry->value = info;

qapi/machine.json

@@ -357,13 +357,16 @@
 # @default-cpu-type: default CPU model typename if none is requested via
 #                    the -cpu argument. (since 4.2)
 #
+# @default-ram-id: the default ID of initial RAM memory backend (since 5.2)
+#
 # Since: 1.2.0
 ##
 { 'struct': 'MachineInfo',
   'data': { 'name': 'str', '*alias': 'str',
             '*is-default': 'bool', 'cpu-max': 'int',
             'hotpluggable-cpus': 'bool',  'numa-mem-supported': 'bool',
-            'deprecated': 'bool', '*default-cpu-type': 'str' } }
+            'deprecated': 'bool', '*default-cpu-type': 'str',
+            '*default-ram-id': 'str' } }
 
 ##
 # @query-machines: