Fix xencommons for fillup

The usage in xen.spec is like this:
 %post tools
 %{fillup_only -n xencommons xencommons}

After fresh install, modify the key=value pairs as required, then
upgrade the package, the sysconfig file is broken and changes are lost:

 # rm /etc/sysconfig/xencommons
 # zypper in --oldpackage xen-tools-4.12.4_02-3.30.1
 # echo XENSTORETYPE=domain >> /etc/sysconfig/xencommons
 # echo XENSTORE_DOMAIN_SIZE=123 >> /etc/sysconfig/xencommons
 # zypper in --oldpackage xen-tools-4.12.4_04-3.33.1
 # diff -u /var/adm/fillup-templates/sysconfig.xencommons /etc/sysconfig/xencommons

Basically fillup removed all comments, and also the two added key=value lines.

Prevent this by defining all keys, with empty values, so that consumers
of the values will continue to use the built-in defaults.
Index: xen-4.16.0-testing/tools/hotplug/Linux/init.d/sysconfig.xencommons.in
===================================================================
--- xen-4.16.0-testing.orig/tools/hotplug/Linux/init.d/sysconfig.xencommons.in
+++ xen-4.16.0-testing/tools/hotplug/Linux/init.d/sysconfig.xencommons.in
@@ -3,7 +3,9 @@
 ## Default: "none"
 #
 # Log xenconsoled messages (cf xl dmesg)
-#XENCONSOLED_TRACE=[none|guest|hv|all]
+# One of [none|guest|hv|all]
+#
+XENCONSOLED_TRACE=
 
 ## Type: string
 ## Default: daemon
@@ -16,10 +18,10 @@
 #
 # Changing this requires a reboot to take effect.
 #
-#XENSTORETYPE=daemon
+XENSTORETYPE=
 
 ## Type: string
-## Default: xenstored
+## Default: @XENSTORED@
 #
 # Select xenstore implementation, this can be either
 # of these below.
@@ -30,7 +32,7 @@
 #  * @sbindir@/xenstored
 #
 # Changing this requires a reboot to take effect.
-#XENSTORED=@XENSTORED@
+XENSTORED=
 
 ## Type: string
 ## Default: unlimited
@@ -57,9 +59,10 @@ XENSTORED_ARGS=
 ## Type: string
 ## Default: Not defined, tracing off
 #
-# Log xenstored messages
+# Log xenstored messages if a non-empty value is assigned.
 # Only evaluated if XENSTORETYPE is "daemon".
-#XENSTORED_TRACE=[yes|on|1]
+#
+XENSTORED_TRACE=
 
 ## Type: integer
 ## Default: 50
@@ -75,14 +78,14 @@ XENSTORED_ARGS=
 #
 # xenstore domain kernel.
 # Only evaluated if XENSTORETYPE is "domain".
-#XENSTORE_DOMAIN_KERNEL=@LIBEXEC@/boot/xenstore-stubdom.gz
+XENSTORE_DOMAIN_KERNEL=
 
 ## Type: integer
 ## Default: 8
 #
 # xenstore domain memory size in MiB.
 # Only evaluated if XENSTORETYPE is "domain".
-#XENSTORE_DOMAIN_SIZE=8
+XENSTORE_DOMAIN_SIZE=
 
 ## Type: string
 ## Default: not set, no autoballooning of xenstore domain
@@ -93,7 +96,7 @@ XENSTORED_ARGS=
 # - combination of both in form of <val>:<frac> (e.g. 8:1/100), resulting
 #   value will be the higher of both specifications
 # Only evaluated if XENSTORETYPE is "domain".
-#XENSTORE_MAX_DOMAIN_SIZE=
+XENSTORE_MAX_DOMAIN_SIZE=
 
 ## Type: string
 ## Default: ""
@@ -106,4 +109,4 @@ XENSTORE_DOMAIN_ARGS=
 #QEMU_XEN=@qemu_xen_path@
 
 # Dom0 UUID
-#XEN_DOM0_UUID=00000000-0000-0000-0000-000000000000
+XEN_DOM0_UUID=