tests: skip virfirewalltest on non-Linux systems
tests: skip virfirewalltest on non-Linux systems

Currently firewalling is supported on Linux only, so skip the
virfirewalltest on other platforms.

VenkatDatta/libvirt,zippy2/libvirt,nertpinx/libvirt,agx/libvirt,crobinso/libvirt,crobinso/libvirt,zippy2/libvirt,fabianfreyer/libvirt,eskultety/libvirt,taget/libvirt,fabianfreyer/libvirt,elmarco/libvirt,jardasgit/libvirt,shugaoye/libvirt,fabianfreyer/libvirt,olafhering/libvirt,rlaager/libvirt,taget/libvirt,rlaager/libvirt,libvirt/libvirt,jfehlig/libvirt,andreabolognani/libvirt,nertpinx/libvirt,andreabolognani/libvirt,nertpinx/libvirt,nertpinx/libvirt,jfehlig/libvirt,andreabolognani/libvirt,jardasgit/libvirt,jardasgit/libvirt,elmarco/libvirt,shugaoye/libvirt,fabianfreyer/libvirt,zippy2/libvirt,nertpinx/libvirt,rlaager/libvirt,taget/libvirt,rlaager/libvirt,libvirt/libvirt,VenkatDatta/libvirt,eskultety/libvirt,datto/libvirt,olafhering/libvirt,taget/libvirt,elmarco/libvirt,datto/libvirt,jfehlig/libvirt,eskultety/libvirt,jfehlig/libvirt,elmarco/libvirt,VenkatDatta/libvirt,agx/libvirt,olafhering/libvirt,crobinso/libvirt,zippy2/libvirt,andreabolognani/libvirt,olafhering/libvirt,eskultety/libvirt,libvirt/libvirt,VenkatDatta/libvirt,jardasgit/libvirt,crobinso/libvirt,rlaager/libvirt,shugaoye/libvirt,datto/libvirt,andreabolognani/libvirt,datto/libvirt,libvirt/libvirt,shugaoye/libvirt,agx/libvirt,taget/libvirt,agx/libvirt,fabianfreyer/libvirt,elmarco/libvirt,shugaoye/libvirt,eskultety/libvirt,VenkatDatta/libvirt,datto/libvirt,agx/libvirt,jardasgit/libvirt