qom: Assert that objects being destroyed have no parent

qom: Assert that objects being destroyed have no parent

QOM reference counting bugs are often hard to detect, but there's
one kind of bug that's easier: if we are freeing an object but is
still attached to a parent, it means the reference count is wrong
(because the parent always hold a reference to their children).

Add an assertion to make sure we detect those cases.

Signed-off-by: Eduardo Habkost <[email protected]>
Message-Id: <20201215224133.3545901[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>