fix #254: iscsi: add support for multipath targets

fix #254: iscsi: add support for multipath targets

With this patch Proxmox now tries to login to all discovered portals
in case some of them are not logged yet.
In case of multipath configuration when initially configured portal is
missing for some reason Proxmox don't lose iSCSI storage now and can
successfully restore iSCSI connection between reboots.

Signed-off-by: Yuri Konotopov <ykonotopov@gnome.org>
Reviewed-By: Dominik Csapak <d.csapak@proxmox.com>
Tested-By: Dominik Csapak <d.csapak@proxmox.com>