qt: Fix regression in QIODeviceOutStream + MSVC

vsnprintf actually works fine than qvsnprintf on MSVC nowadays so use
that
7 jobs for msvc2 in 5 minutes and 4 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #6704508
build

00:04:35

passed #6704513
build_android

00:00:46

passed #6704509
build_clang_libcpp

00:05:03

passed #6704512
build_clazy

00:04:15

passed #6704511
build_mingw64_fedora

00:03:14

passed #6704510
build_ubuntu_16_04

00:01:45

passed #6704507
clang_format

00:01:06