qom: trace asserting casts

This provides a way to detect the cast that leads to a (reproducible)
crash even when QOM cast debugging is disabled.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1368188203-3407-6-git-send-email-pbonzini@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
