pci: fix double free of romfile property

The qdev property release function frees any string properties.  This was
resulting in a double free during hot unplug.

It manifests in network devices because block devices have a NULL romfile
property by default.

Cc: Michael Tsirkin <mst@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/pci.c

@@ -841,7 +841,6 @@ static int pci_unregister_device(DeviceState *dev)
 
     pci_unregister_io_regions(pci_dev);
     pci_del_option_rom(pci_dev);
-    g_free(pci_dev->romfile);
     do_pci_unregister_device(pci_dev);
     return 0;
 }