backends/igvm: Handle policy for SEV guests

backends/igvm: Handle policy for SEV guests

Adds a handler for the guest policy initialization IGVM section and
builds an SEV policy based on this information and the ID block
directive if present. The policy is applied using by calling
'set_guest_policy()' on the ConfidentialGuestSupport object.

Signed-off-by: Roy Hopkins <roy.hopkins@randomman.co.uk>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Stefano Garzarella <sgarzare@redhat.com>
Acked-by: Gerd Hoffman <kraxel@redhat.com>
Reviewed-by: Ani Sinha <anisinha@redhat.com>
Link: https://lore.kernel.org/r/57707230bef331b53e9366ce6a23ed25cd6f1293.1751554099.git.roy.hopkins@randomman.co.uk
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>