sysinit: Accept also old xenbus kernel interface

qubes-sysinit.sh waits for xenbus initialization by watching its
interface file presence. In linux before 3.10 there is no
/dev/xen/xenbus, which is the case in Debian 7 (3.2 kernel). The problem
applies only to the VMs with PVGrub enabled, because otherwise VM would
use dom0 privided kernel, which is much newer.

Fixes QubesOS/qubes-issues#1609

vm-systemd/qubes-sysinit.sh

@@ -23,8 +23,8 @@ if ! dmesg | grep -q "$systemd_pkg_version running in system mode."; then
     systemctl daemon-reexec
 fi
 
-# Wait for evtchn initialization
-while [ ! -e /dev/xen/xenbus ]; do
+# Wait for xenbus initialization
+while [ ! -e /dev/xen/xenbus -a ! -e /proc/xen/xenbus ]; do
   sleep 0.1
 done