maint: Change Guix System file names from guixsd-*.

* Makefile.am (GUIXSD_SUPPORTED_SYSTEMS): Rename to...
(GUIX_SYSTEM_SUPPORTED_SYSTEMS): ... this.
(GUIXSD_VM_SYSTEMS): Rename to...
(GUIX_SYSTEM_VM_SYSTEMS): ... this.
(GUIXSD_IMAGE_BASE): Rename to...
(GUIX_SYSTEM_IMAGE_BASE): ... this.  Change "guixsd-" to "guix-system-".
(GUIXSD_VM_IMAGE_BASE): Rename to...
(GUIX_SYSTEM_VM_IMAGE_BASE): ... this.  Change "guixsd-" to
"guix-system-".
(release): Adjust accordingly.
* doc/guix.texi (USB Stick and DVD Installation)
(Installing Guix in a VM, Invoking guix system)
(Running Guix in a VM): Adjust file names accordingly.