The word voronka means "a funnel" (and has no relation to vorona, "a crow"). So "voronochka" could possibly be interpreted as "little funnel," which is probably not a good nickname for a girlfriend. But as nsdfrv says, crows/ravens don't have highly positive associations in Russian, even though scientists say they're among the most intelligent of birds.

If your girlfriend's real name starts with "V" and you want something phonetically similar, I agree that "vorobushek" seems better. (The Roman poet Catullus wrote a rather famous poem about the death of his girlfriend's pet sparrow, and wished that she would hold him -- I mean Catullus -- against her breasts, as she had once held the sparrow. So sparrows have had lovey-dovey symbolism since classical times.)