Nova recently introduced support of SEV-ES, to extend the existing
memory encryption support using SEV. Add the new metadef value to
explain the flavor extra spec and the image property to determine
the hardware/CPU feature for memory encryption.
Also fix the outdated link of nova documentation about AMD SEV support.
Depends-on: https://review.opendev.org/c/openstack/nova/+/926106
Change-Id: I9063663a3b14161a67da59a68f5f7c92fd5447f5
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>