xen pvfb: Module aliases to support module autoloading

These are mostly for completeness and consistency with the other
frontends, as PVFB is typically compiled in rather than a module.

Derived from
http://xenbits.xensource.com/linux-2.6.18-xen.hg?rev/5e294e29a43e



While there, add module descriptions.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>