libxl: MigratePerform: properly cleanup after libxlDomObjFromDomain

libxlDomObjFromDomain to returns locked and ref counted virDomainObj but
libxlDomainMigratePerform3Params only unlocks the object on exit. Convert
it to use the virDomainObjEndAPI function for cleanup.
Signed-off-by: NJim Fehlig <jfehlig@suse.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>

src/libxl/libxl_driver.c

@@ -6097,8 +6097,7 @@ libxlDomainMigratePerform3Params(virDomainPtr dom,
     ret = 0;
 
  cleanup:
-    if (vm)
-        virObjectUnlock(vm);
+    virDomainObjEndAPI(&vm);
     return ret;
 }