Add double serializer to handle String and int values

Add double serializer to handle String and int values

Adding a custom serializer for double values allows us to handle
situations in which the PVE API provides values in other types. The most
likely possibility is that numbers are formatted as string in the JSON
response which needs to be parsed to double.

We must also handle the situation that the value is an integer which
needs to be cast to double.

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>