qom: object_delete should unparent the object first

qom: object_delete should unparent the object first

object_deinit is only called when the reference count goes to zero,
and yet tries to do an object_unparent.  Now, object_unparent
either does nothing or it will decrease the reference count.
Because we know the reference count is zero, the object_unparent
call in object_deinit is useless.

Instead, we need to disconnect the object from its parent just
before we remove the last reference apart from the parent's.  This
happens in object_delete.  Once we do this, all calls to
object_unparent peppered through QEMU can go away.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>