vmm/vmd: add exception injection and refactor inject api.

vmm/vmd: add exception injection and refactor inject api.

In order to continue work on mmio and other instruction emulation,
vmd(8) needs the ability to inject exceptions (like page faults)
from userland.

Refactor the way events are injected from userland, cleaning up how
hardware (external) interrupts are injected in the process.

ok mlarkin@