virtio: drop a useless config read

    "virtio: core support for config generation"
fixed reading up 64 bit values, adding generation
checks for such reads.

By mistake, it left an explicit get call in place
as well. the result is that the value is read twice,
the first result is discarded.

Not a big deal since this only happens with virtio
blk and only on boot ATM, so performance isn't
affected, but let's clean it up.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>