KVM: selftests: Move platform_info_test's main assert into guest code

KVM: selftests: Move platform_info_test's main assert into guest code

As a first step toward gracefully handling the expected #GP on RDMSR in
platform_info_test, move the test's assert on the non-faulting RDMSR
result into the guest itself.  This will allow using a unified flow for
the host userspace side of things.

Link: https://lore.kernel.org/r/20240314232637.2538648-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>