frontend: Refactor Vm Behavior classes

Utilize the builders infrastructure to reduce code duplication in
mappings from VmBase to UnitVmModel in classes derived from
VmModelBehaviorBase.

To write proper unit tests it was necessary to modify the
ConstantsManager class to encapsulate all code invoking GWT.create().

Change-Id: I10a67a8b63e2c8c0eda0f59c88be11dadbe9872e
Signed-off-by: Martin Betak <mbetak@redhat.com>