Rework snapshot code, has_feature

Rework snapshot code, has_feature

Move snapshot_create, snapshot_delete and snapshot_rollback
into abstract pve-common/src/PVE/AbstractConfig.pm,
splitting LXC-specific parts into __snapshot_XX methods in
src/PVE/LXC/Config.pm.

check_freeze_needed, snapshot_prepare and snapshot_commit
are downgraded to private __snapshot_XX methods (in
PVE::AbstractConfig and PVE::LXC::Config).

has_feature is made an implementation of the abstract
has_feature, and thus moves into src/PVE/LXC/Config.pm