block: add support for encryption secrets in block I/O tests

The LUKS block driver tests will require the ability to specify
encryption secrets with block devices. This requires using the
--object argument to qemu-img/qemu-io to create a 'secret'
object.

When the IMGKEYSECRET env variable is set, it provides the
password to be associated with a secret called 'keysec0'

The _qemu_img_wrapper function isn't modified as that needs
to cope with differing syntax for subcommands, so can't be
made to use the image opts syntax unconditionally.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 1462896689-18450-3-git-send-email-berrange@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
