config: parse_volume: don't die when noerr is set

config: parse_volume: don't die when noerr is set

AFAICT, the only existing callers using noerr=1 are in
__snapshot_delete_remove_drive, and in AbstractConfig's
foreach_volume_full. The former should not be affected, as unknown
keys should never make their way in there. For the latter, it makes
iterating with
    $opts = { extra_keys => ['vmstate'] }
possible while being agnostic of guest type. Previously, it would die
for LXC configs, but now the unknown key is simply skipped there.

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