KVM: x86: Disallow changing MSR_PLATFORM_INFO after vCPU has run

KVM: x86: Disallow changing MSR_PLATFORM_INFO after vCPU has run

Tag MSR_PLATFORM_INFO as a feature MSR (because it is), i.e. disallow it
from being modified after the vCPU has run.

To make KVM's selftest compliant, simply delete the userspace MSR write
that restores KVM's original value at the end of the test.  Verifying that
userspace can write back what it originally read is uninteresting in this
particular case, because KVM doesn't enforce _any_ bits in the MSR, i.e.
userspace should be able to write any arbitrary value.

Link: https://lore.kernel.org/r/20240802185511.305849-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>