core: change the visibility of validation method in RunVmValidator

The visibility of validation methods in RunVmValidator that should not
be invoked from outside of this class is changed to 'protected'. Those
are the validation methods that gets all the data they need for their
checks as parameters (inversion of control principle, to make them more
testable).

Change-Id: Iee1ef7ed2f812cf929328c8219ed95e98d2959ae
Signed-off-by: Arik Hadas <ahadas@redhat.com>