api: config: use shared guesthelpers in GET call

api: config: use shared guesthelpers in GET call

since containers can also have pending changes now, we need a method to
get the current applied config as well as the one with the pending
changes inside. this makes the GET config api more consistent with
qemu-server's by reusing load_current_config and load_snapshot_config from
AbstractConfig.
to decide which method to call, we look at the parameters.

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