xenfront: initialize softc structure

The Xen hypervisor not always initialize its pages. The BSD drivers will
however assume they are zeroed, and so things may break if they are not
(misterious flag tests suddenly being true, for instance).

Initialize manually the data we have just received from malloc when we deal
with softc.

drivers/xenfront.cc

@@ -75,6 +75,7 @@ void xenfront_driver::set_ivars(struct xenbus_device_ivars *ivars)
     _bsd_dev.softc = malloc(table->size);
     // Simpler and we don't expect driver loading to fail anyway
     assert(_bsd_dev.softc);
+    memset(_bsd_dev.softc, 0, table->size);
 }
 
 void xenfront_driver::finished()