nsis installer: Fix mouse-over descriptions for emulators

nsis installer: Fix mouse-over descriptions for emulators

We use the nsis.py script to write out an installer script Section
for each emulator executable, so the exact set of Sections depends on
which executables were built.  However the part of qemu.nsi which
specifies mouse-over descriptions for each Section still has a
hard-coded and very outdated list (with just i386 and alpha).  This
causes two problems.  Firstly, if you build the installer for a
configuration where you didn't build the i386 binaries you get
warnings like this:
  warning 6000: unknown variable/constant "{Section_i386}" detected, ignoring (macro:_==:1)
  warning 6000: unknown variable/constant "{Section_i386w}" detected, ignoring (macro:_==:1)
(this happens in our gitlab CI jobs, for instance).
Secondly, most of the emulators in the generated installer don't have
any mouseover text.

Make nsis.py generate a second output file which has the necessary
MUI_DESCRIPTION_TEXT lines for each Section it creates, so we can
include that at the right point in qemu.nsi to set the mouse-over
text.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 20220305105743.2384766-4-peter.maydell@linaro.org