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Provide some mitigation against CVE-2017-5715 by clearing registers

Description

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:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=5b6c02f38315b720c593c6079364855d276886aa

Reviewed by: grehan, rgrimes
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D13573

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