KVM: selftests: Drop 'int' return from asserting *_has_device_attr()

Drop 'int' returns from *_device_has_attr() helpers that assert the
return is '0', there's no point in returning '0' and "requiring" the
caller to perform a redundant assertion.

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