Older versions of gcc (eg 4.6) can't handle varargs functions declared
inline for anything other than completely trivial uses, and complain:

tests/qom-test.c: In function 'qmp': tests/libqtest.h:359:60: sorry,
unimplemented: function 'qmp' can never be inlined because it uses
variable argument lists

Avoid this problem by putting the functions into libqtest.c instead
of using inline definitions in libqtest.h.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NAndreas Färber <afaerber@suse.de>