[virt-tools-list] Generate a nsis installer when building mingw-virt-viewer.spec

[Christophe Fergeau]

Hey,

This patch series aims at making it simpler to update the Windows builds of
remote-viewer, after these changes, mingw-virt-viewer.spec + a virt-viewer
tarball + mock or koji are enough to automatically get a nsis installer
binary in the end (wrapped in a .rpm).
A side effect of these changes is that mingw-virt-viewer.spec will now
build a gtk+2 version of virt-viewer, but this is more consistent with
the fact that the .nsis file we ship is expecting a gtk+2 build.

Christophe