tests/virtio-blk-test: read config space after feature negotiation

tests/virtio-blk-test: read config space after feature negotiation

The VIRTIO Configuration Space cannot be accessed before device feature
bits have been read because a driver doesn't know the endianness until
it has checked VIRTIO_F_VERSION_1.

Fix this problem in preparation for VIRTIO 1.0 support.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20191023100425.12168-2-stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>