vm: Formalize use of '-virtfs' options.

* gnu/system/vm.scm (file-system->mount-tag, host-9p-file-system): New
  procedures.
  (virtualized-operating-system): Use 'host-9p-file-system' for the
  store.
  (common-qemu-options): Add 'shared-fs' parameter.
  [virtfs-option]: New procedure.
  Use it.
  (system-qemu-image/shared-store-script): Adjust accordingly.