xen: use our own eventchannel->irq path

Rather than overloading vectors for event channels, take full
responsibility for mapping an event channel to irq directly.  With
this patch Xen has its own irq allocator.

When the kernel gets an event channel upcall, it maps the event
channel number to an irq and injects it into the normal interrupt
path.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>