Implement cancellation method for dma async I/O (Avi Kivity)

Implement cancellation method for dma async I/O (Avi Kivity)

Move the dma helpers to a private aio pool, and implement a cancellation
method for them.  Should prevent issues when cancelling I/O while dma is
in progress.

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@6872 c046a42c-6fe2-441c-8c8c-71466251a162
(cherry picked from commit 6512a2a7106480c19183d6466a6845bc9bdf6ec0)
(cherry picked from commit 798e5374e78fe11dc8d1993b2318756f222878dd)