* 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.