MFC r328011,329162

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.

git-svn-id: svn://svn.freebsd.org/base/stable/10@330713 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f