qapi: fix visitor serialization tests for numbers/doubles

We never actually stored the stringified double values into the strings
before we did the comparisons. This left number/double values completely
uncovered in test-visitor-serialization tests.

Fixing this exposed a bug in our handling of large whole number values
in QEMU's JSON parser which is now fixed.

Simplify the code while we're at it by dropping the
calc_float_string_storage() craziness in favor of GStrings.

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Amos Kong <akong@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
