vmm: emulate INVD by ignoring it

On physical systems the ram isn't initialized on boot. So, coreboot uses
the cache as ram in this boot phase. When exiting cache as ram, coreboot
calls INVD for making the cache consistent.

In a virtual environment ram is always initialized and the cache is
always consistent. So, we can safely ignore this call.

Reviewed by: jhb, imp
Differential Revision: https://reviews.freebsd.org/D35620
Sponsored by: Beckhoff Automation GmbH & Co. KG