virtio-blk: Fix virtio-blk-s390 to require drive

Move the check from virtio_blk_init_pci(), where it protects only
virtio-blk-pci, to virtio_blk_init().  Without that, virtio-blk-s390
initializes without a drive.  I figure that can lead to null pointer
dereferences.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
