[PATCH 0 of 4] aio event fd support to blktap2

[Daniel Stodden <daniel.stodden@citrix.com>]

Get blktap2 running on pvops.

This mainly adds eventfd support to the userland code. Based on some
prior cleanup to tapdisk-queue and the server object. We had most of
that in XenServer for a while, so I kept it stacked.

1. Clean up IPC and AIO init in tapdisk-server. 

   [I think tapdisk-ipc in blktap2 is basically obsolete.
    Pending a later patch to remove it?]

2. Split tapdisk-queue into variable raw I/O backends.

   This basically makes an 'ops'-struct (struct tio) out of what used
   to be primarily libaio vs. an if/else hack to resort to canonical
   read()/write()s where desirable.

   For now, the one chosen remains as hardcoded as ever.

3. Prefer AIO eventfd support on kernels >= 2.6.22

   Mainline Linux after 2.6.22 finally got I/O muxing for
   AIO. Unfortunately, few systems bring the necessary libaio update
   (0.3.107), xen/tools included.

   Since this is just about a bunch of inline macros and an update to
   reserved space in the iocb struct, let's add a compat header with
   private typedefs instead.

   This should obsolete tools/aio.

Misc:
  - Does a runtime kernel version check. I guess this code will
    need additional cpp magic on BSDs.
  - Wants a PERROR macro in blktaplib.h
  - Fixes a bug in tapdisk-vbd which locks up the sync io mode.
  - Removed dead code in qcow2raw to make it link again.