qom: Detect attempts to add a property that already exists

qom: Detect attempts to add a property that already exists

Detect attempts to add a property to an object if one of
that name already exists, and report them as errors.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1364217314-7400-2-git-send-email-peter.maydell@linaro.org
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>