backends/confidential-guest-support: Add set_guest_policy() function

backends/confidential-guest-support: Add set_guest_policy() function

For confidential guests a policy can be provided that defines the
security level, debug status, expected launch measurement and other
parameters that define the configuration of the confidential platform.

This commit adds a new function named set_guest_policy() that can be
implemented by each confidential platform, such as AMD SEV to set the
policy. This will allow configuration of the policy from a
multi-platform resource such as an IGVM file without the IGVM processor
requiring specific implementation details for each platform.

Signed-off-by: Roy Hopkins <roy.hopkins@randomman.co.uk>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Ani Sinha <anisinha@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Gerd Hoffman <kraxel@redhat.com>
Link: https://lore.kernel.org/r/d3888a2eb170c8d8c85a1c4b7e99accf3a15589c.1751554099.git.roy.hopkins@randomman.co.uk
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>