storage: better validate iSCSI portal info
storage: better validate iSCSI portal info

Ensure that the iSCSI portal exposes at least a valid target,
otherwise loop till the user enters a valid configuration.

Change-Id: I73a5bff75fb4f24124a5ebb6c575ae0bd4c7d0db
Bug-Url: https://bugzilla.redhat.com/1487560
Signed-off-by: Simone Tiraboschi <cb03129c66ba4da7ba7044049d0f43907bf857bb@redhat.com>

oVirt/ovirt-hosted-engine-setup,oVirt/ovirt-hosted-engine-setup