core: some cleanup in ImportVmCommand

- Replace AddCanDoActionMessage calls and 'return false' right after
  them with calls to failCanDoAction
- Refactor templateExistsOnExportDomain method to reduce the number of
  nested levels in it
- Rename templateExistsOnExportDomain to isTemplateExistsOnExportDomain
- Refactor updateSnapshotsFromExport method to reduce the number of
  nested levels in it
- Simplified canAddVm method by removing the 'exists' field

Change-Id: Ibfdb7bba0758cfccb55bc237f22659cb1993cc00
Signed-off-by: Arik Hadas <ahadas@redhat.com>