Cope with missing swap cgroup controls

It is possible to build a kernel without swap cgroup controls
present. This causes a fatal error when querying memory
parameters. Treat missing swap controls as meaning "unlimited".
The fatal error remains if the user tries to actually change
the limit.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

src/qemu/qemu_driver.c

@@ -7613,9 +7613,12 @@ qemuDomainGetMemoryParameters(virDomainPtr dom,
         case 2: /* fill swap hard limit here */
             rc = virCgroupGetMemSwapHardLimit(priv->cgroup, &val);
             if (rc != 0) {
-                virReportSystemError(-rc, "%s",
-                                     _("unable to get swap hard limit"));
-                goto cleanup;
+                if (rc != -ENOENT) {
+                    virReportSystemError(-rc, "%s",
+                                         _("unable to get swap hard limit"));
+                    goto cleanup;
+                }
+                val = VIR_DOMAIN_MEMORY_PARAM_UNLIMITED;
             }
             if (virTypedParameterAssign(param,
                                         VIR_DOMAIN_MEMORY_SWAP_HARD_LIMIT,