KVM: selftests: Split vcpu_set_nested_state() into two helpers

Split vcpu_nested_state_set() into a wrapper that asserts, and an inner
helper that does not.  Passing a bool is all kinds of awful as it's
unintuitive for readers and requires returning an 'int' from a function
that for most users can never return anything other than "success".

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>