conf: Avoid formatting empty redirfilter element
conf: Avoid formatting empty redirfilter element

If the redirfilter has no usbdev sub-elements, then do not format anything
rather than formatting an empty pair of elements:

    <redirfilter>
    </redirfilter>

Signed-off-by: Luyao Huang <23d9981432efa88691fd2a71f5525bc2174f7664@redhat.com>

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