HostGeneralModel: improve "node has upgrade" logic

This patch will add a validation to compare Node iso version against
ISOs available in oVirt Engine. If ISOs on Engine are higher,
then show to users that we have new upgrade available.

Change-Id: I49c74d3d0d92c74848e0c175095610b6e6b5a88e
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=911347
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>