x86/msr: Fix migration compatibility issue with MSR_SPEC_CTRL

x86/msr: Fix migration compatibility issue with MSR_SPEC_CTRL

This bug existed in early in 2018 between MSR_SPEC_CTRL arriving in microcode,
and SSBD arriving a few months later.  It went unnoticed presumably because
everyone was busy rebooting everything.

The same bug will reappear when adding PSFD support.

Clamp the guest MSR_SPEC_CTRL value to that permitted by CPUID on migrate.
The guest is already playing with reserved bits at this point, and clamping
the value will prevent a migration to a less capable host from failing.

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