Blueprint add-kvm-hidden-feature added the capability of hiding the kvm
signature from guests. However, it was implemented only through an image
property.
A major reason for this feature is to allow passed-through Nvidia GPUs
to work correctly. GPU pci-passthrough is specified on the flavor's
extra_specs, without requiring an image with special properties.
Therefore, hiding the KVM signature should also be specifiable through
the flavor's extra_specs, in order to not require a special image for
this use case.
If the new flavor extra_spec is present and set to 'true', the libvirt
driver will produce an additional element to hide kvm's signature on
the vm, in the same way as with the image property
`img_hide_hypervisor_id`.
Implements: blueprint hide-hypervisor-id-flavor-extra-spec
Closes-Bug: 1757424
Change-Id: I41c5913b4148629b448ea5fb43b7597dc067dc22