List iSCSI server for initiator while creating iSCSI Pool

The server should be already used in an existed iSCSI Pool.

Test this patch by:
$ curl -k -u <user>:<password> -H "Content-Type: application/json" \
> -H "Accept: application/json" \
> https://localhost:8001/storageservers?_target_type=iscsi
[
  {
    "host":"127.0.0.1"
    "port":"3260"
  }
]

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
List iSCSI targets available for initiator while creating iSCSI Pool

Here we call libvirt findStoragePoolSources to get the iSCSI targets.

Test this patch by:
$ curl -k -u <user>:<password> -H "Content-Type: application/json" \
> -H "Accept: application/json" \
> https://localhost:8001/storageservers/127.0.0.1/storagetargets?_target_type=iscsi\&_server_port=3260
[
  {
    "host":"127.0.0.1",
    "target":"iqn.2003-01.org.linux-iscsi.localhost.x8664:sn.edb1a004dc57",
    "target_type":"iscsi"
  }
]

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>