migration: Deprecate block migration

It is obsolete.  It is better to use driver-mirror with NBD instead.

CC: Kevin Wolf <kwolf@redhat.com>
CC: Eric Blake <eblake@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Hanna Czenczek <hreitz@redhat.com>

Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231018115513.2163-5-quintela@redhat.com>