Datagram socket is available since qemu 4.0, commit
fdec16e3c2a614e2861f3086b05d444b5d8c3406 ("net/socket: learn to talk
with a unix dgram socket").

Required for slirp-helper communication.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>