Do not use /proc/xen for detecting dom0 anymore

Phase out /proc/xen usage. Relevant device files are available in
/dev/xen. Dom0 check can be replaced with uuid check - dom0 have
well-known value of all-0.

QubesOS/qubes-issues#2540

initramfs-tools/qubes_vm

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-if grep -q control_d /proc/xen/capabilities; then
+if grep -q '^[0-]*$' /sys/hypervisor/uuid; then
     echo "Not intended for dom0"
     exit 0
 fi