destroy: remove pending volumes as well

destroy: remove pending volumes as well

if a volume is only referenced in the pending section of a config it was
previously not removed when removing the CT, unless the non-default
'remove unreferenced disks' option was enabled.

keeping track of volume IDs which we attempt to remove gets rid of false
warnings in case a volume is referenced both in the config and the
pending section, or multiple times in the config for other reasons.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>