block-vbd: implement support for splitting requests

block-vbd: implement support for splitting requests

the main issue with stubdoms at the moment is that the new dma api does
not support an upper limit for the number of sectors in a single dma
transfer.
This means that block-vbd can issue blkfront_aio transfers that exceed
the blktap limit BLKIF_MAX_SEGMENTS_PER_REQUEST.
The attached qemu patch makes block-vbd split any request bigger than
the limit above.

The small xen patch is also needed to change struct blkfront_aiocb.
 [ The corresponding changeset in xen-unstable is 19463:f2cf89a4e762 -iwj ]

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>