hostmem: separate allocation from UserCreatable complete method

This allows the superclass to set various policies on the memory
region that the subclass creates. Drops hostmem-ram's complete method
accordingly.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

backends/hostmem-ram.c

@@ -16,9 +16,8 @@
 
 
 static void
-ram_backend_memory_init(UserCreatable *uc, Error **errp)
+ram_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
 {
-    HostMemoryBackend *backend = MEMORY_BACKEND(uc);
     char *path;
 
     if (!backend->size) {
@@ -35,9 +34,9 @@ ram_backend_memory_init(UserCreatable *uc, Error **errp)
 static void
 ram_backend_class_init(ObjectClass *oc, void *data)
 {
-    UserCreatableClass *ucc = USER_CREATABLE_CLASS(oc);
+    HostMemoryBackendClass *bc = MEMORY_BACKEND_CLASS(oc);
 
-    ucc->complete = ram_backend_memory_init;
+    bc->alloc = ram_backend_memory_alloc;
 }
 
 static const TypeInfo ram_backend_info = {

backends/hostmem.c

@@ -75,11 +75,31 @@ host_memory_backend_get_memory(HostMemoryBackend *backend, Error **errp)
     return memory_region_size(&backend->mr) ? &backend->mr : NULL;
 }
 
+static void
+host_memory_backend_memory_complete(UserCreatable *uc, Error **errp)
+{
+    HostMemoryBackend *backend = MEMORY_BACKEND(uc);
+    HostMemoryBackendClass *bc = MEMORY_BACKEND_GET_CLASS(uc);
+
+    if (bc->alloc) {
+        bc->alloc(backend, errp);
+    }
+}
+
+static void
+host_memory_backend_class_init(ObjectClass *oc, void *data)
+{
+    UserCreatableClass *ucc = USER_CREATABLE_CLASS(oc);
+
+    ucc->complete = host_memory_backend_memory_complete;
+}
+
 static const TypeInfo host_memory_backend_info = {
     .name = TYPE_MEMORY_BACKEND,
     .parent = TYPE_OBJECT,
     .abstract = true,
     .class_size = sizeof(HostMemoryBackendClass),
+    .class_init = host_memory_backend_class_init,
     .instance_size = sizeof(HostMemoryBackend),
     .instance_init = host_memory_backend_init,
     .instance_finalize = host_memory_backend_finalize,

include/sysemu/hostmem.h

@@ -34,6 +34,8 @@ typedef struct HostMemoryBackendClass HostMemoryBackendClass;
  */
 struct HostMemoryBackendClass {
     ObjectClass parent_class;
+
+    void (*alloc)(HostMemoryBackend *backend, Error **errp);
 };
 
 /**