abstract config: add method to calculate derived properties from a config

abstract config: add method to calculate derived properties from a config

HA manager currently needs to know about internal details about the
configs and how the properties are calculated. With this method, those
details are abstracted away, allowing to change the configuration
structure. In particular, QemuConfig's 'memory' can be turned into
a property string without HA manager needing to know about it (once HA
manager switched to using this mehtod).

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