qcow2: add iotests to cover LUKS encryption support

This extends the 087 iotest to cover LUKS encryption when doing
blockdev-add.

Two further tests are added to validate read/write of LUKS
encrypted images with a single file and with a backing file.

Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170623162419.26068-15-berrange@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
