KVM: selftests: Check result in hyperv_features for successful hypercalls

Commit cc5851c6 ("KVM: selftests: Use exception fixup for #UD/#GP
Hyper-V MSR/hcall tests") introduced a wrong guest assert in guest_hcall().
It is not checking the successful hypercall results and only checks the
result when a fault happens.

  GUEST_ASSERT_2(!hcall->ud_expected || res == hcall->expect,
                 hcall->expect, res);

Correct the assertion by only checking results of the successful
hypercalls.

This issue was observed when this test started failing after building it
in Clang. Above guest assert statement fails because "res" is not equal
to "hcall->expect" when "hcall->ud_expected" is true. "res" gets some
garbage value in Clang from the RAX register. In GCC, RAX is 0 because
it using RAX for @output_address in the asm statement and resetting it
to 0 before using it as output operand in the same asm statement. Clang
is not using RAX for @output_address.

Fixes: cc5851c6 ("KVM: selftests: Use exception fixup for #UD/#GP Hyper-V MSR/hcall tests")
Signed-off-by: Vipin Sharma <vipinsh@google.com>
Suggested-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20220922062451.2927010-1-vipinsh@google.com


[sean: wrap changelog at ~75 chars, move -EFAULT change to separate patch]
Signed-off-by: Sean Christopherson <seanjc@google.com>