core: move network validations on run vm to RunVmValidator

Move the network validations from RunVmCommand to RunVmValidator, where
this kind of validations should reside. The methods that were moved were
changed to return ValidationResult instead of boolean to conform the
standard.

Change-Id: I6aff70b45617e90deac826f2a873c1604c9a4241
Signed-off-by: Arik Hadas <ahadas@redhat.com>