xen pvfb: Zero unused bytes in events sent to backend

This isn't a security flaw (the backend can see all our memory
anyway).  But it's the right thing to do all the same.

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>