Documentation: virt: correct location of haltpoll module params

Module parameters are located in sysfs, not debugfs, so correct the
statement.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Link: https://lore.kernel.org/r/20230610054302.6223-1-rdunlap@infradead.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>