x86/spec-ctrl: Introduce an option to control L1TF mitigation for PV guests

x86/spec-ctrl: Introduce an option to control L1TF mitigation for PV guests

Shadowing a PV guest is only available when shadow paging is compiled in.
When shadow paging isn't available, guests can be crashed instead as
mitigation from Xen's point of view.

Ideally, dom0 would also be potentially-shadowed-by-default, but dom0 has
never been shadowed before, and there are some stability issues under
investigation.

This is part of XSA-273 / CVE-2018-3620.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>