avoid list context for volume_size_info calls

avoid list context for volume_size_info calls

With the recent pve-storage commit d70d814 ("api: fix get content call response
type for RBD/ZFS/iSCSI volumes"), the volume_size_info call for RBD in
list context is much slower than before (from a quick test, about twice as long
without snapshots, even longer with snapshots and untested, but when using an
external cluster with image not having the fast-diff feature, it should be worse
still) and thus increases the likelihood to run into timeouts here.

None of the callers here actually need the more expensive call, so just
avoid calling in list context.

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