KVM: selftests: assert on exit reason in CR4/cpuid sync test

...so that the test doesn't end up in an infinite loop if it fails for
whatever reason, e.g. SHUTDOWN due to gcc inserting stack canary code
into ucall() and attempting to derefence a null segment.

Fixes: ca359066 ("kvm: selftests: add cr4_cpuid_sync_test")
Cc: Wei Huang <wei@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>