docs/system: Add documentation on support for IGVM

docs/system: Add documentation on support for IGVM

IGVM support has been implemented for Confidential Guests that support
AMD SEV and AMD SEV-ES. Add some documentation that gives some
background on the IGVM format and how to use it to configure a
confidential guest.

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: Pankaj Gupta <pankaj.gupta@amd.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Gerd Hoffman <kraxel@redhat.com>
Reviewed-by: Ani Sinha <anisinha@redhat.com>
Link: https://lore.kernel.org/r/b4dc920a30717e19cd79bbbe2cc769f3b9ff3d37.1751554099.git.roy.hopkins@randomman.co.uk
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>