fix #4085: properly activate cicustom storage(s)

fix #4085: properly activate cicustom storage(s)

PVE::Storage::path() neither activates the storage of the passed-in volume, nor
does it ensure that the returned value is actually a file or block device, so
this actually fixes two issues. PVE::Storage::abs_filesystem_path() actually
takes care of both, while still calling path() under the hood (since $volid
here is always a proper volid, unless we change the cicustom schema at some
point in the future).

Reviewed-by: Fiona Ebner <f.ebner@proxmox.com>
Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>