object: check strong flag with &

The following patch is going to introduce more flags.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20200110153039.1379601-13-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>