frontend: Generify RunOnceModel & VmInitModel

Utilize generic EntityModels in RunOnceModel and VmInitModel and
associated widgets.

In order to generify the ComboBox class used int both models also the
VolumeParameterModel was generified to be consistent with the new
generic ComboBox usage.

Also removed now unused deprecated EntityModelTextArea and
EntityModelTextAreaEditor.

Change-Id: I378b2151bda422ec5997cc82cbeb3ef24f940416
Signed-off-by: Martin Betak <mbetak@redhat.com>