Until now we disabled the submit button, based on
the feature API call where we tested if the selected VM, and its snapshot
can be used as the source of a clone.
This had the following problems:
* the feature test for copy clone / linked clone always returned true,
since the passed paramaters were already filtered before user input
(ie we only allowed a linked clone to be selected if the source VM is
a template, or a snasphot to be selected if existed on source VM)
* the guest ID input field was not validated
With the current patch, the validation is now made on each of the form
fields. verifyFeature() is still used to populate a list of valid nodes
passed to the Node Selector.
This allows to display a 'CT', a 'VM', or a 'CT/VM' label
to be displayed, depending on the context.
Currently when restoring a CT backup or creating a CT via the
wizard, we are asked to enter a *VM* ID, which is confusing.
