libxl: Add refcnt for args->vm during migration

libxl: Add refcnt for args->vm during migration

When adding the @vm to the @args for usage during a thread or
callback, let's add the reference to it at the time of adding to
ensure nothing else deletes it. The corresponding Unref is then
added to the Dispose function.

Signed-off-by: John Ferlan <jferlan@redhat.com>
Reviewed-by: Marc Hartmayer <mhartmay@linux.vnet.ibm.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>