dracut: add a safety check preventing qubes_cow_setup.sh running in dom0

Theoretically it shouldn't be a problem, because module isn't installed
in initramfs by default (in dom0), but since such error would be fatal
to dom0 (will prevent it from booting), add a safety check for it.

Probably it will be required when (if) we migrate dom0 to Debian

dracut/full-dmroot/qubes_cow_setup.sh

@@ -41,6 +41,11 @@ fi
 
 info "Qubes initramfs script here:"
 
+if ! grep -q 'root=[^ ]*dmroot' /proc/cmdline; then
+    warn "dmroot not requested, probably not a Qubes VM"
+    exit 0
+fi
+
 if [ -e /dev/mapper/dmroot ] ; then 
     die "Qubes: FATAL error: /dev/mapper/dmroot already exists?!"
 fi