[HVM] Replace shared-memory PIC state with a set-irq-level hypercall.

[HVM] Replace shared-memory PIC state with a set-irq-level hypercall.
This simplifies the IRQ logic significantly and avoids the bogus
hvm_pic_assist() on domain resume path.

There is more work to be done here. At least:
 1. set-irq-level should really be set-interrupt-wire-level. Wire
    state needs to be distinguished from PIC (in particular, PIC IRR)
    state.
 2. Hypercalls can be batched in qemu and pushed down in one
    multicall.

Signed-off-by: Keir Fraser <keir@xensource.com>