KVM: x86: expose MSR_PLATFORM_INFO as a feature MSR

For userspace that wants to disable KVM_X86_QUIRK_STUFF_FEATURE_MSRS, it
is useful to know what bits can be set to 1 in MSR_PLATFORM_INFO (apart
from the TSC ratio).  The right way to do that is via /dev/kvm's
feature MSR mechanism.

In fact, MSR_PLATFORM_INFO is already a feature MSR for the purpose of
blocking updates after the vCPU is run, but KVM_GET_MSRS did not return
a valid value for it.

Just like in a VM that leaves KVM_X86_QUIRK_STUFF_FEATURE_MSRS enabled,
the TSC ratio field is left to 0.  Only bit 31 is set.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>