Do not error when qvm-get-default-netvm returns empty string.

It happens when installing qubes-core-dom0 for the first time.

dom0/init.d/qubes_core

@@ -46,6 +46,9 @@ stop()
 {
     echo -n $"Shutting down all Qubes VMs:"
     NETVM=$(qvm-get-default-netvm)
+    if [ "X"$NETVM = "X" ] ; then
+        NETVM="dom0"
+    fi
     qvm-run -q --shutdown --all --wait --exclude $NETVM
     rm -f /var/lock/subsys/qubes_core
     killall meminfo-writer

rpm_spec/core-dom0.spec

@@ -175,7 +175,7 @@ done
 /etc/init.d/qubes_core start
 
 NETVM=$(qvm-get-default-netvm)
-if [ $NETVM = "dom0" ] ; then
+if [ "X"$NETVM = "X""dom0" ] ; then
     /etc/init.d/qubes_netvm start
 fi
 
@@ -191,7 +191,7 @@ fi
 if [ "$1" -gt 1 ] ; then
     # upgrading already installed package...
     NETVM=$(qvm-get-default-netvm)
-    if [ $NETVM = "dom0" ] ; then
+    if [ "X"$NETVM = "X""dom0" ] ; then
         /etc/init.d/qubes_netvm stop
     fi