utils_test.libvirt: Fix pre_pool for iscsi_get_sessions.

I adopt @will-Do 's idea to recover tuple of get_iscsi_sessions.
Because, the destroy failed of iscsi-pool caused by not suitable ip_addr, 
so we should use what we get from sessions. 
@jferlan, you can try this one now.

I have left the port in returns of sessions for extension in future when we need that port.

 * Let iscsi.get_iscsi_sessions return tuple of target and address
 * Before cleanup of pool in PoolVolumeTest, we should check
   whether pool exists.
 * As iscsi.iscsi_get_sessions returns ip_addr and port,
   we should remove port for pool-operations.