qom: Demote already-has-a-parent to a regular error

qom: Demote already-has-a-parent to a regular error

Rather than an abort(). This allows callers to decide whether parenting
an already-parented object is a fatal error condition.

Useful for providing a default value for an object's parent in the case
where you want to set one iff it doesn't already have one.

Reviewed-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>