pci: memory leak of PCIDevice::rom_file

PCIDevice::rom_file is leaked.
PCIDevice::rom_file is allocated in pci_qdev_init(), but not freed anywhere.
free it in qemu_unregister_device().

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
