qemu: domain: Extract code to determine topmost nodename to qemuDomainDiskGetTopNodename

There are more places which require getting the topmost nodename to be
passed to qemu. Separate it out into a new function.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

src/qemu/qemu_domain.c

@@ -11530,6 +11530,31 @@ qemuDomainDetermineDiskChain(virQEMUDriverPtr driver,
 }
 
 
+/**
+ * qemuDomainDiskGetTopNodename:
+ *
+ * @disk: disk definition object
+ *
+ * Returns the pointer to the node-name of the topmost layer used by @disk as
+ * backend. Currently returns the nodename of the copy-on-read filter if enabled
+ * or the nodename of the top image's format driver. Empty disks return NULL.
+ * This must be used only when VIR_QEMU_CAPS_BLOCKDEV is enabled.
+ */
+const char *
+qemuDomainDiskGetTopNodename(virDomainDiskDefPtr disk)
+{
+    qemuDomainDiskPrivatePtr priv = QEMU_DOMAIN_DISK_PRIVATE(disk);
+
+    if (virStorageSourceIsEmpty(disk->src))
+        return NULL;
+
+    if (disk->copy_on_read == VIR_TRISTATE_SWITCH_ON)
+        return priv->nodeCopyOnRead;
+
+    return disk->src->nodeformat;
+}
+
+
 /**
  * qemuDomainDiskGetBackendAlias:
  * @disk: disk definition
@@ -11549,8 +11574,6 @@ qemuDomainDiskGetBackendAlias(virDomainDiskDefPtr disk,
                               virQEMUCapsPtr qemuCaps,
                               char **backendAlias)
 {
-    qemuDomainDiskPrivatePtr priv = QEMU_DOMAIN_DISK_PRIVATE(disk);
-    const char *nodename = NULL;
     *backendAlias = NULL;
 
     if (!virQEMUCapsGet(qemuCaps, QEMU_CAPS_BLOCKDEV)) {
@@ -11560,16 +11583,7 @@ qemuDomainDiskGetBackendAlias(virDomainDiskDefPtr disk,
         return 0;
     }
 
-    if (virStorageSourceIsEmpty(disk->src))
-        return 0;
-
-    if (disk->copy_on_read == VIR_TRISTATE_SWITCH_ON)
-        nodename = priv->nodeCopyOnRead;
-    else
-        nodename = disk->src->nodeformat;
-
-    *backendAlias = g_strdup(nodename);
-
+    *backendAlias = g_strdup(qemuDomainDiskGetTopNodename(disk));
     return 0;
 }
 

src/qemu/qemu_domain.h

@@ -874,6 +874,10 @@ int qemuDomainStorageFileInit(virQEMUDriverPtr driver,
                               virStorageSourcePtr parent);
 char *qemuDomainStorageAlias(const char *device, int depth);
 
+const char *
+qemuDomainDiskGetTopNodename(virDomainDiskDefPtr disk)
+    ATTRIBUTE_NONNULL(1);
+
 int qemuDomainDiskGetBackendAlias(virDomainDiskDefPtr disk,
                                   virQEMUCapsPtr qemuCaps,
                                   char **backendAlias)