guest: make call_host_function generic to avoid two steps to retrieve return value

[dblnz]

Description

As the title says, this changes the call_host_function function to return a generic type.

Details

The reason behind this change is to avoid calling the host function and getting the result in a separate step.
For the C guests we still need to do it in two steps as it does not support generic types.
To avoid code duplication I added a call_host_function_internal that does what call_host_function used to. In addition I added call_host_function<T> which calls the call_host_function_internal and also retrieves the return value.

This approach allows us to use a generic function to call and retrieve the return value in Rust and we can use call_host_function_internal for C guests.

[ludfjig]

LGTM, nice improvement