lxc: Refresh capabilities if they have never been initalized

Adjust virLXCDriverGetCapabilities to fill in driver->caps if it is
empty, regardless of the passed 'refresh' value. This matches the
pattern used in virQEMUDriverGetCapabilities

This fixes LXC XML startup parsing for me

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Cole Robinson <crobinso@redhat.com>

src/lxc/lxc_conf.c

@@ -196,6 +196,13 @@ virCapsPtr virLXCDriverGetCapabilities(virLXCDriverPtr driver,
         driver->caps = caps;
     } else {
         lxcDriverLock(driver);
+
+        if (driver->caps == NULL) {
+            VIR_DEBUG("Capabilities didn't detect any guests. Forcing a "
+                      "refresh.");
+            lxcDriverUnlock(driver);
+            return virLXCDriverGetCapabilities(driver, true);
+        }
     }
 
     ret = virObjectRef(driver->caps);