abstractconfig: add load_current_config and load_snapshot_config

abstractconfig: add load_current_config and load_snapshot_config

this code is already used by qemu-server's GET config API call. it is
however better to split this into to methods and decide what to run in
the API call.

this general implementation uses some $class helpers which allow to abstract
away the difference in the child classes. this will be used for
containers once they can do pending changes.

Signed-off-by: Oguz Bektas <o.bektas@proxmox.com>