hw/pci: fixed hotplug crash when using rombar=0 with devices having romfile

Hot-plugging a device that has a romfile (either supplied by user
or built-in) using rombar=0 option is a user error,
do not allow the device to be hot-plugged.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Marcel Apfelbaum <marcel.a@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
