mark PBS storages as shared

mark PBS storages as shared

Like this, the property will get added when parsing the storage configuration
and PBS storages will correctly show up as shared storages in API results.

AFAICT the only affected PBS operation is free_image via vdisk_free, which will
now be protected by a cluster-wide lock, and that shouldn't hurt.

Another issue this fixes, which is the reason this patch exists, was reported
in the forum[0]. The free space from PBS storages was counted once for each node
that had access to the storage.

[0]: https://forum.proxmox.com/threads/pve-6-3-the-storage-size-was-displayed-incorrectly.83136/

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>