qemu: Disable NBD TLS migration over UNIX socket

qemu: Disable NBD TLS migration over UNIX socket

Even though it is technically possible, when running the migrations QEMU's
nbd-server-start errors out with:

  "TLS is only supported with IPv4/IPv6"

We can always enable it when QEMU adds this feature, but for now it is safer to
show our error message rather than rely on QEMU to error out properly.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>