maint: Provide the configuration file in the VM image.

This fixes a bug introduced in 362bcdb1b0
whereby the VM image would no longer contain /etc/config.scm, contrary
to what the manual says.

Reported by dbdude on #guix.

* Makefile.am (release): Pass '--save-provenance' to 'guix system
vm-image'.
* doc/guix.texi (Running Guix in a VM): Adjust file name accordingly.

Makefile.am

@@ -818,6 +818,7 @@ release: dist-with-updated-version
 	for system in $(GUIX_SYSTEM_VM_SYSTEMS) ; do					\
 	  image=`$(top_builddir)/pre-inst-env						\
 	    guix system vm-image $(GUIX_SYSTEM_VM_IMAGE_FLAGS)				\
+	    --save-provenance								\
 	    --system=$$system --fallback						\
 	    gnu/system/examples/vm-image.tmpl` ;					\
 	  if [ ! -f "$$image" ] ; then							\

doc/guix.texi

@@ -27252,7 +27252,8 @@ This image boots the Xfce graphical environment and it contains some
 commonly-used tools.  You can install more software in the image by running
 @command{guix package} in a terminal (@pxref{Invoking guix package}).  You can
 also reconfigure the system based on its initial configuration file available
-as @file{/etc/config.scm} (@pxref{Using the Configuration System}).
+as @file{/run/current-system/configuration.scm} (@pxref{Using the
+Configuration System}).
 
 Instead of using this pre-built image, one can also build their own virtual
 machine image using @command{guix system vm-image} (@pxref{Invoking guix