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: Anthony Liguori <aliguori@us.ibm.com>
