[XEN][HVM] Make sure that the interrupt which event channel events come in

[XEN][HVM] Make sure that the interrupt which event channel events come in
on is level triggered rather than edge triggered, since it's a PCI device.
This is complicated by the possibility that another PCI device could be
on the same interrupt; the workaround is to have two irr registers
for the PIC and APIC, and have qemu and Xen generated interrupts go
into different ones.

This broke the alt_irq stuff.  Fortunately, nobody uses that anymore, so
I've removed it.

Signed-off-by: Steven Smith <sos22@cam.ac.uk>