engine: Only iSCSI storage connections are viable for iSCSI Bond

iSCSI Bond is the new managed entity introduced in 3.4 in order to
configure iSCSI multipathing. This entity is configured based on iSCSI
storage connections therefore only connections of that type should be
presented to the user when it configures the iSCSI Bond in UI.

Change-Id: Iae30db6aa0c5d2989e9e31566d6555d70259799e
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>