Nova has never supported direct booting of an image of an encrypted
volume uploaded to Glance via the Cinder upload-volume-to-image
process, but instead of rejecting such a request, an 'active' but
unusable instance is created. This patch allows Nova to use image
metadata to detect such an image and reject the boot request.
Change-Id: Idf84ccff254d26fa13473fe9741ddac21cbcf321
Related-bug: #1852106
Closes-bug: #1863611
(cherry picked from commit 963fd8c0f9)