Refactor mountpoint and general conf methods

Refactor mountpoint and general conf methods

Move add_unused_volume into abstract
pve-common/src/PVE/AbstractConfig.pm, because it is
identical for LXC and Qemu.

Move classify_mountpoint, is_volume_in_use, has_dev_console,
mountpoint_names, foreach_mountpoint_XX and get_vm_volumes
to PVE::LXC::Config because they only deal with config
related matters.

(Some of) the latter methods might get moved to or become
implementations of methods in PVE::AbstractConfig in the
future.