maint: Provide the configuration file in the VM image.

This fixes a bug introduced in 362bcdb1
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.