libxl: Resolve Coverity CHECKED_RETURN

Add a check of the return for virDomainHostdevInsert() like every
other call.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>

src/libxl/libxl_driver.c

@@ -2891,7 +2891,8 @@ libxlDomainAttachDeviceConfig(virDomainDefPtr vmdef, virDomainDeviceDefPtr dev)
                 return -1;
             }
 
-            virDomainHostdevInsert(vmdef, hostdev);
+            if (virDomainHostdevInsert(vmdef, hostdev) < 0)
+                return -1;
             break;
 
         default: