userportal: refactor in UserPortalListModel & BootSequenceModel

- extract the code that set the boot from network option for a given vm
  from UserPortalListModel#RunOnce to a new private method to make it a
  little bit shorter
- extract the common code in BootSequenceModel#getHardDiskOption,
  BootSequenceModel#getNetworkOption, BootSequenceModel#getCdromOption
  to a new private method to reduce the code duplication

Change-Id: I841bedf241e82f6f89e88a66d6cd4e4ea681102f
Signed-off-by: Arik Hadas <ahadas@redhat.com>