hw/pci-host: pnv_phb{3, 4}: Fix heap out-of-bound access failure

pnv_phb3_root_bus_info and pnv_phb4_root_bus_info are missing the
instance_size initialization. This results in accessing out-of-bound
memory when setting 'chip-id' and 'phb-id', and eventually crashes
glib's malloc functionality with the following message:

  "qemu-system-ppc64: GLib: ../glib-2.72.3/glib/gmem.c:131: failed to allocate 3232 bytes"

This issue was noticed only when running qtests with QEMU Windows
32-bit executable. Windows 64-bit, Linux 32/64-bit do not expose
this bug though.

Fixes: 9ae1329e ("ppc/pnv: Add models for POWER8 PHB3 PCIe Host bridge")
Fixes: 4f9924c4 ("ppc/pnv: Add models for POWER9 PHB4 PCIe Host bridge")
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Xuzhou Cheng <xuzhou.cheng@windriver.com>
Signed-off-by: Bin Meng <bin.meng@windriver.com>
Message-Id: <20220920103159.1865256-29-bmeng.cn@gmail.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>