Fix the way to get iscsi sessions

The old way(regex) may get no result, such as:
$iscsiadm -m session
tcp: [74] [fe80::4637:e6ff:fe4a:42d1%eth0]:3260,1 iqn.2001-01.com.virttest:test-pool.target (non-flash)

So discard the regex and use string split to get the right result.

Signed-off-by: Yanbing Du <ydu@redhat.com>