core: extract balloon validation in import vm

It is better to extract the validation from the canDoAction command for
2 reasons:
1. Readability - to make the canDoAction method more readable
2. Testability - to make the command more testable

Change-Id: I152a9b96e9804ba66bc967f2754a33c346222674
Signed-off-by: Arik Hadas <ahadas@redhat.com>