Add support for level triggered interrupt pins on the vioapic. Prior to this

Add support for level triggered interrupt pins on the vioapic. Prior to this
commit level triggered interrupts would work as long as the pin was not shared
among multiple interrupt sources.

The vlapic now keeps track of level triggered interrupts in the trigger mode
register and will forward the EOI for a level triggered interrupt to the
vioapic. The vioapic in turn uses the EOI to sample the level on the pin and
re-inject the vector if the pin is still asserted.

The vhpet is the first consumer of level triggered interrupts and advertises
that it can generate interrupts on pins 20 through 23 of the vioapic.

Discussed with:	grehan@