Using new method to setup iscsi device

The new method wrap dependency checking, iscsi target setup and login steps,
and set default values for all needed params.

BTW, this patch solve the problem mentioned in https://github.com/autotest/tp-libvirt/pull/286, for ```virsh_attach_detach_disk.py```, which already fix in https://github.com/autotest/tp-libvirt/pull/193/, and for ```virtual_disks_multidisks.py```, which will be fixed by @rbian in his patches.

Signed-off-by: Yanbing Du <ydu@redhat.com>