Refactor aio callback allocation to use an aiocb pool (Avi Kivity)

Refactor aio callback allocation to use an aiocb pool (Avi Kivity)

Move the AIOCB allocation code to use a dedicate structure, AIOPool.  AIOCB
specific information, such as the AIOCB size and cancellation routine, is
moved into the pool.

At present, there is exactly one pool per block format driver, maintaining
the status quo.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6870 c046a42c-6fe2-441c-8c8c-71466251a162

[ Backported from 6bbff9a0b495918309074ac60375be5f9dc868b3
  by Stefano Stabellini. ]

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
(cherry picked from commit efb4162ee6ad7ea15d1589eb09409854379f5139)