vm: Introduce 'file-system-mapping'.

* gnu/system/vm.scm (<file-system-mapping>): New record type.
  (%store-mapping): New variable.
  (host-9p-file-system): Rename to...
  (mapping->file-system): ... this.  Replace 'source' and 'target'
  parameters with 'mapping'.  Set 'flags' field.
  (virtualized-operating-system): Add 'mappings' parameter and  honor
  it.
  (system-qemu-image/shared-store-script): Add 'mappings' parameter.
  Pass it to 'virtualized-operating-system'.  Use it in argument to
  'common-qemu-options'.