[tgtadm] Update target faile when backup volume

[TommyLike]

This could happen with the newly installed devstack environment.

0. This is my environment:
```
tgtadm --version
1.0.43
uname -a
Linux SZX1000159695 3.13.0-24-generic #47-Ubuntu SMP Fri May 2
23:30:00 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
```

1. show targets with command: tgtadm --lld iscsi --op show --mode target

output:
```
Target 1: iqn.2010-10.org.openstack:volume-8e414689-990c-4545-bfd7-435cef858513
    System information:
        Driver: iscsi
        State: ready
    I_T nexus information:
    LUN information:
        LUN: 0
            Type: controller
            SCSI ID: IET 00010000
            SCSI SN: beaf10
            Size: 0 MB, Block size: 1
            Online: Yes
            Removable media: No
            Prevent removal: No
            Readonly: No
            SWP: No
            Thin-provisioning: No
            Backing store type: null
            Backing store path: None
            Backing store flags:
    Account information:
        c5guKcoR7xUjS3KZdzmg
    ACL information:
        ALL

```

2. update target without force option command (if -f option is given,
everything seems good):sudo tgt-admin -u
iqn.2010-10.org.openstack:volume-8e414689-990c-4545-bfd7-435cef858513
-v
output:
```
# default-driver not defined, defaulting to iscsi.

# Removing target:
iqn.2010-10.org.openstack:volume-8e414689-990c-4545-bfd7-435cef858513
tgtadm -C 0 --mode target --op delete --tid=1

# Adding target:
iqn.2010-10.org.openstack:volume-8e414689-990c-4545-bfd7-435cef858513
tgtadm -C 0 --lld iscsi --op new --mode target --tid 1 -T
iqn.2010-10.org.openstack:volume-8e414689-990c-4545-bfd7-435cef858513

```
Then the shell stuck here and get no response. Is this a correct
behaviour?