qemu: Make sure preferredMachines is not missing any entry
qemu: Make sure preferredMachines is not missing any entry

With the current implementation, adding a new architecture
and not updating preferredMachines accordingly will not
cause a build failure, making it very likely that subtle
bugs will be introduced in the process. Rework the code
so that such issues will be caught by the compiler.

Signed-off-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>
Reviewed-by: Erik Skultety <2c14d38fa47c8799f1b9c16280abe27f8edfec6e@redhat.com>

nertpinx/libvirt,olafhering/libvirt,jfehlig/libvirt,andreabolognani/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,olafhering/libvirt,jardasgit/libvirt,eskultety/libvirt,eskultety/libvirt,andreabolognani/libvirt,zippy2/libvirt,fabianfreyer/libvirt,crobinso/libvirt,crobinso/libvirt,jardasgit/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,zippy2/libvirt,nertpinx/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,libvirt/libvirt,libvirt/libvirt,jfehlig/libvirt,eskultety/libvirt,libvirt/libvirt,nertpinx/libvirt,andreabolognani/libvirt,jfehlig/libvirt,olafhering/libvirt,eskultety/libvirt,jardasgit/libvirt,zippy2/libvirt,libvirt/libvirt,zippy2/libvirt,crobinso/libvirt,andreabolognani/libvirt,jardasgit/libvirt,crobinso/libvirt,eskultety/libvirt,olafhering/libvirt,nertpinx/libvirt