Disable the submit button based on whole form validity

Disable the submit button based on whole form validity

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.