If slirp is disabled, it will fail with:

qemu-system-x86_64: -netdev user,id=qtest-bn0: Parameter 'type' expects a netdev backend type
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>