core: minor refactoring in RunVmValidator

- rename methods which are validating network related stuff to being
  with 'validate'
- moved CanRunVm to the top of the class
- organized the class methods order such that the validation methods
  would be grouped together and the utility method be in the buttom

Change-Id: I80ac05cb4028578364ef710603a674834a33a1a2
Signed-off-by: Arik Hadas <ahadas@redhat.com>