domain_conf: make virDomainVirtioNetGuestOpts/HostOpts/DriverFormat() void
domain_conf: make virDomainVirtioNetGuestOpts/HostOpts/DriverFormat() void

These functions always return zero, so they might as well be void.

Signed-off-by: Matt Coleman <1fa2ef4755a9226cb9a0a4840bd89b158ac71391@datto.com>
Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

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