virt: vmgenid: move to using dev_set/get_drvdata

The prior commit moved the order of initializing driver_data around. In
looking through the tree at what is normally done, it appears that
actually few drives set or get driver_data directly, but instead go
through the dev_set/get_drvdata helpers, which are simple inline helpers
that amount to the same exact code. So, for the sake of consistency, use
the helpers.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>