virtio-pci: Check for virtio_blk_init() failure

It can't actually fail now, but the next commit will change that.

s390_virtio_blk_init() already checks for failure, but
virtio_blk_init_pci() doesn't.  Fix that.

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