fix #3161: snapshot creation: only check volumes for fsfreeze

fix #3161: snapshot creation: only check volumes for fsfreeze

When considering mountpoints for running 'fsfreeze' before snapshot
creation, commit 8463099d99273561c46398bf02206b4d9d431bc5 did not
only consider volumes created by our storage-stack, but also
bindmounts and devmounts (directly mounting a blockdevice).

This led to PVE::Storage::parse_volume_id failing on those
mountpoints.

Since the fsfreeze call is best-effort and only run for specific
storageplugins, we can simply skip non-volume mountpoints, when
gathering the list of volumes to call fsfreeze on.

Tested with a container with a bind mount.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Tested-by: Dominic Jäger <d.jaeger@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>