driver: allow drivers to indicate if they permit remote connections

driver: allow drivers to indicate if they permit remote connections

Add a localOnly flag to the virConnectDriver struct which allows a
driver to indicate whether it is local-only, or permits remote
connections. Stateful drivers running inside libvirtd are generally
local only. This allows us to remote the check for uri->server != NULL
from most drivers.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>