Several functions in server/ were not specifying an argument list,
ie they were declared as void foo(); When compiling with
-Wstrict-prototypes, this leads to:
test_playback.c:93:5: erreur: function declaration isn’t a prototype
[-Werror=strict-prototypes]
When compiling spice with make CFLAGS="-g3 -ggdb3 -O0 -Wall -Werror",
the build broken because of a few unused variables/missing returns.
This patch fixes these warnings.