KVM: selftests: Load RAX with -EFAULT before Hyper-V hypercall

KVM: selftests: Load RAX with -EFAULT before Hyper-V hypercall

Load RAX with -EFAULT prior to making a Hyper-V hypercall so that tests
can't get false negatives due to the compiler coincidentally loading the
"right" value into RAX, i.e. to ensure that _KVM_ and not the compiler
is correctly clearing RAX on a successful hypercall.

Note, initializing *hv_status (in C code) to -EFAULT is not sufficient
to avoid false negatives, as the compiler can still "clobber" RAX and
thus load garbage into *hv_status if the hypercall faults (or if KVM
doesn't set RAX).

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Vipin Sharma <vipinsh@google.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20220922062451.2927010-1-vipinsh@google.com
[sean: move to separate patch, massage changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>