Provide some mitigation against CVE-2017-5715 by clearing registers
upon returning from the guest which aren't immediately clobbered by
the host. This eradicates any remaining guest contents limiting their
usefulness in an exploit gadget.
This was inspired by this linux commit:
Reviewed by: grehan, rgrimes
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D13573