[Improvement] backup TOTAL-SIZE units remain consistent

[linghan-hub]

kbcli version
Kubernetes: v1.25.14-eks-f8587cb
KubeBlocks: 0.7.0-beta.10
kbcli: 0.7.0-beta.10

k get backup
NAME                                 POLICY                                METHOD            REPO      STATUS      TOTAL-SIZE   DURATION   CREATION-TIME          COMPLETION-TIME        EXPIRATION-TIME
backup-mysql-cluster                 mysql-cluster-mysql-backup-policy     xtrabackup        my-repo   Completed   4435229      38s        2023-10-24T06:42:38Z   2023-10-24T06:43:16Z   2023-10-31T06:43:16Z
backup-mysql-cluster-snapshot        mysql-cluster-mysql-backup-policy     volume-snapshot   my-repo   Completed   20Gi         42s        2023-10-24T07:01:45Z   2023-10-24T07:02:28Z   2023-10-31T07:02:28Z
backup-postgresql-cluster-snapshot   pg-cluster-postgresql-backup-policy   volume-snapshot   my-repo   Completed   20Gi         16s        2023-10-24T07:11:45Z   2023-10-24T07:12:01Z   2023-10-31T07:12:01Z