virttest.qemu_storage: Check if session already present before login

Check if the session already present in host before login it in setup
for iscsi devices. Sometimes the host env is not clean and we want to
leave a warning message in log file instead of raising an error.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
virttest.storage: Add init timeout for iscsidev

OS need several seconds to init the iscsi device under /dev after
login to the target. Wait until this is finished when we try to
get the device name for iscsidev. The default timeout is set to
10s.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #1566 from ypu/iscsi

Iscsi small bugs fix.