hyperv/vmbus: Complete vmbus initialization; interrupt cputimer is enabled

hyperv/vmbus: Complete vmbus initialization; interrupt cputimer is enabled

Most of the bits are obtained from FreeBSD.  However, The interrupt bits
are reworked:
- Since the vmbus message/event interrupt works in the same fashion as
  MSI-X, we just allocate MSI-X for them, instead of allocating IDT
  vector, rolling vmbus own interrupt vector and turning the interrupt
  handling inside-out.  The standard and generic bus APIs are used to
  allocate and setup per-cpu vmbus interrupt.
- Interrupt cputimer reuses the current per-cpu interrupt timer code.
- AutoEOI is not used, since we reuse the per-cpu interrupt timer IDT
  vector and MSI IDT vector.  After a brief discussion w/ Dexuan Cui,
  I concluded that AutoEOI probably does not provide noticible performance
  improvement but will introduce extra code complexity.  We leave it off
  for now.

Obtained-from: FreeBSD (mostly)