When using config.h, it must be the very first include in all source
files since it contains #define that may change the compilation process
(eg libc structure layout changes when it's used to enable large file
support on 32 bit x86 archs). This commit adds it at the beginning
of all .c and .cpp files
With this function you can update an added uint32 after it being added.
To make this possible all the spice_marshaller_add_add_foo functions
now return a pointer that can be used as a reference when later
setting a value.
