sev: Update launch_update_data functions to use Error handling

sev: Update launch_update_data functions to use Error handling

The class function and implementations for updating launch data return
a code in case of error. In some cases an error message is generated and
in other cases, just the error return value is used.

This small refactor adds an 'Error **errp' parameter to all functions
which consistently set an error condition if a non-zero value is
returned.

Signed-off-by: Roy Hopkins <roy.hopkins@randomman.co.uk>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Gerd Hoffman <kraxel@redhat.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Ani Sinha <anisinha@redhat.com>
Link: https://lore.kernel.org/r/d59721f7b99cfc87aab71f8f551937e98e983615.1751554099.git.roy.hopkins@randomman.co.uk
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>