qemu: hotplug: Fix the condition check for net->downscript
qemu: hotplug: Fix the condition check for net->downscript

According to the context, here we are checking net->downscript's validity,

Signed-off-by: Liao Pingfang <62afbc03024ea40b8236face99b5bba86187d267@zte.com.cn>
Signed-off-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>
Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

olafhering/libvirt,jfehlig/libvirt,jfehlig/libvirt,crobinso/libvirt,jardasgit/libvirt,nertpinx/libvirt,olafhering/libvirt,jardasgit/libvirt,libvirt/libvirt,zippy2/libvirt,nertpinx/libvirt,nertpinx/libvirt,libvirt/libvirt,libvirt/libvirt,nertpinx/libvirt,olafhering/libvirt,crobinso/libvirt,jfehlig/libvirt,olafhering/libvirt,crobinso/libvirt,jardasgit/libvirt,jardasgit/libvirt,crobinso/libvirt,zippy2/libvirt,jardasgit/libvirt,nertpinx/libvirt,jfehlig/libvirt,zippy2/libvirt,libvirt/libvirt,zippy2/libvirt