storage: backend: Use correct stringifier for pool type

When registering a storage poll backend, the code would use
virStorageTypeToString instead of virStoragePoolTypeToString. The
following message would be logged:

virDriverLoadModuleFunc:71 : Lookup function 'virStorageBackendSCSIRegister'
virStorageBackendRegister:174 : Registering storage backend '(null)'
(cherry picked from commit 894133a3)

src/storage/storage_backend.c

@@ -171,12 +171,12 @@ int
 virStorageBackendRegister(virStorageBackendPtr backend)
 {
     VIR_DEBUG("Registering storage backend '%s'",
-              virStorageTypeToString(backend->type));
+              virStoragePoolTypeToString(backend->type));
 
     if (virStorageBackendsCount >= VIR_STORAGE_BACKENDS_MAX) {
         virReportError(VIR_ERR_INTERNAL_ERROR,
                        _("Too many drivers, cannot register storage backend '%s'"),
-                       virStorageTypeToString(backend->type));
+                       virStoragePoolTypeToString(backend->type));
         return -1;
     }