qemu: domain: Add helper for getting the disk backend alias

The disk backend alias was historically the alias of the -drive backing
the storage. For setups with -blockdev this will become more complex as
it will depend on other configs and generally will differ.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

src/qemu/qemu_domain.c

@@ -8599,6 +8599,34 @@ qemuDomainDetermineDiskChain(virQEMUDriverPtr driver,
 }
 
 
+/**
+ * qemuDomainDiskGetBackendAlias:
+ * @disk: disk definition
+ * @qemuCaps: emulator capabilities
+ * @backendAlias: filled with the alias of the disk storage backend
+ *
+ * Returns the correct alias for the disk backend. This may be the alias of
+ * -drive for legacy setup or the correct node name for -blockdev setups.
+ *
+ * @backendAlias may be NULL on success if the backend does not exist
+ * (disk is empty). Caller is responsible for freeing @backendAlias.
+ *
+ * Returns 0 on success, -1 on error with libvirt error reported.
+ */
+int
+qemuDomainDiskGetBackendAlias(virDomainDiskDefPtr disk,
+                              virQEMUCapsPtr qemuCaps ATTRIBUTE_UNUSED,
+                              char **backendAlias)
+{
+    *backendAlias = NULL;
+
+    if (!(*backendAlias = qemuAliasDiskDriveFromDisk(disk)))
+        return -1;
+
+    return 0;
+}
+
+
 /**
  * qemuDomainDiskChainElementRevoke:
  *

src/qemu/qemu_domain.h

@@ -732,6 +732,12 @@ int qemuDomainStorageFileInit(virQEMUDriverPtr driver,
                               virStorageSourcePtr parent);
 char *qemuDomainStorageAlias(const char *device, int depth);
 
+int qemuDomainDiskGetBackendAlias(virDomainDiskDefPtr disk,
+                                  virQEMUCapsPtr qemuCaps,
+                                  char **backendAlias)
+    ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2)
+    ATTRIBUTE_NONNULL(3) ATTRIBUTE_RETURN_CHECK;
+
 void qemuDomainDiskChainElementRevoke(virQEMUDriverPtr driver,
                                       virDomainObjPtr vm,
                                       virStorageSourcePtr elem);