MFC r328011,329162

r328011:

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.

r329162:

Provide further mitigation against CVE-2017-5715 by flushing the
return stack buffer (RSB) upon returning from the guest.