vhost-user-blk: Make sure to set Error on realize failure

We have to set errp before jumping to virtio_err, otherwise the caller
(virtio_device_realize()) will take this as success and crash when it
later tries to access things that we've already freed in the error path.

Fixes: 77542d43
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20210429171316.162022-2-kwolf@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Acked-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>