Description of problem:
When adding volume in 'Add volume to boot from' modal and selecting InstanceType while forgetting choosing also size (and being able to click on Save anyway), the appropriate label is not set at all, simply ignored, without letting the user know what's going on.

Version-Release number of selected component (if applicable):
4.13

How reproducible:
always

Steps to Reproduce:
1. Go to Catalog > InstanceTypes tab, or to Bootable volumes list
2. Click on Add volume button
3. Fill in the required fields,
then also select optional 'Default InstanceType' from the drop down,
forget to choose also 'Size'
4. Click on 'Save' button
5. Check the volume's/DataSource's yaml or its Details page, check the labels

Actual results:
'instancetype.kubevirt.io/default-instancetype' is totally missing in the DataSource resource, even when user selected some from 'Default InstanceType' drop down. User has no idea why, what's going on, no idea about the need of choosing also Size when choosing some InstanceType (that is optional for now, btw, when adding new bootable volume).

Expected results:
Some default size (or simply the 1st from the drop down) should be automatically selected once the user chooses 'Default InstanceType' in the modal, so then the InstanceType could be properly set when saving the data. Or some other scenario, other than the actual one. For example, user could be somehow forced to set also the size, if ever choosing some InstanceType. Save button could be disabled till the user chooses also the size, but the user should be informed what is missing.

Additional info:
In Bootable volumes list in Edit modal, there are the same fields available for editing (Default InstanceType, Preference, Description) but the problem IS NOT reproducible there, because it was implemented the way that once the user chooses some InstanceType, the 1st option from sizes is selected by default, too, and the user can change that if they want, of course.
According to mentioned above and for consistency reasons, I'd suggest implementing the same behavior also in Add volume to boot from modal for InstanceType field/drop down. Another benefit of doing so is that we can save some unnecessary clicks for the user, in case the user would be satisfied with the 1st option from available sizes, instead of forcing the user to explicitly choose some size.