KVM: selftests: Rework platform_info_test to actually verify #GP

Rework platform_info_test to actually handle and verify the expected #GP
on RDMSR when the associated KVM capability is disabled.  Currently, the
test _deliberately_ doesn't handle the #GP, and instead lets it escalated
to a triple fault shutdown.

In addition to verifying that KVM generates the correct fault, handling
the #GP will be necessary (without even more shenanigans) when a future
change to the core KVM selftests library configures the IDT and exception
handlers by default (the test subtly relies on the IDT limit being '0').

Link: https://lore.kernel.org/r/20240314232637.2538648-7-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>