guix system: Add '--share' and '--expose' options for 'vm'.

* guix/scripts/system.scm (system-derivation-for-action): Add #:mappings
  parameter.  Pass it to 'system-qemu-image/shared-store-script'.
  (perform-action): Likewise.
  (show-help): Document --share and --expose.
  (specification->file-system-mapping): New procedure.
  (%options): Add --share and --expose.
  (guix-system): Pass #:mapping to 'perform-action'.
* doc/guix.texi (Invoking guix system): Document it.