utils_test.libvirt: Fix pre_pool for iscsi_get_sessions.

 * 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.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Signed-off-by: John Ferlan <jferlan@redhat.com>
Signed-off-by: Yanbing Du <ydu@redhat.com>
Merge pull request #1576 from yumingfei/iscsi_fix

utils_test.libvirt: Fix pre_pool for iscsi_get_sessions.