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