Test migrating from a VM with a persistent bitmap in the backing chain,
and then continuing that VM after the migration

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200730120234.49288-3-mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>