block: support sha256 fingerprint with pre-blockdev options

block: support sha256 fingerprint with pre-blockdev options

When support for sha256 fingerprint checking was aded in

  commit bf783261f0aee6e81af3916bff7606d71ccdc153
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Tue Jun 22 12:51:56 2021 +0100

    block/ssh: add support for sha256 host key fingerprints

it was only made to work with -blockdev. Getting it working with
-drive requires some extra custom parsing.

Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>