driver: introduce a driver method for probing default URIs

Currently the virDrvConnectOpen method is supposed to handle both
opening an explicit URI and auto-probing a driver if no URI is
given. Introduce a dedicated virDrvConnectURIProbe method to enable the
probing functionality to be split from the driver opening functionality.

It is still possible for NULL to be passed to the virDrvConnectOpen
method after this change, because the remote driver needs special
handling to enable probing of the URI against a remote libvirtd daemon.

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

src/driver-hypervisor.h

@@ -25,6 +25,9 @@
 #  error "Don't include this file directly, only use driver.h"
 # endif
 
+typedef int
+(*virDrvConnectURIProbe)(char **uri);
+
 typedef virDrvOpenStatus
 (*virDrvConnectOpen)(virConnectPtr conn,
                      virConnectAuthPtr auth,
@@ -1300,6 +1303,7 @@ typedef virHypervisorDriver *virHypervisorDriverPtr;
  */
 struct _virHypervisorDriver {
     const char *name; /* the name of the driver */
+    virDrvConnectURIProbe connectURIProbe;
     virDrvConnectOpen connectOpen;
     virDrvConnectClose connectClose;
     virDrvConnectSupportsFeature connectSupportsFeature;