Only allow the UNIX transport in remote driver when setuid

Only allow the UNIX transport in remote driver when setuid

We don't know enough about quality of external libraries used
for non-UNIX transports, nor do we want to spawn external
commands when setuid. Restrict to the bare minimum which is
UNIX transport for local usage. Users shouldn't need to be
running setuid if connecting to remote hypervisors in any
case.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
(cherry picked from commit e22b0232c7b94aefaef87c52c4d626fa532fcce3)