ppc/pnv: Use QOM hierarchy to scan PEC PHB4 devices

ppc/pnv: Use QOM hierarchy to scan PEC PHB4 devices

When -nodefaults is supported for PHB4 devices, the pecs array under
the chip will be empty. This will break the 'info pic' HMP command.

Do a QOM loop on the chip children and look for PEC PHB4 devices
instead.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20211213132830.108372-15-clg@kaod.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>