scsi-disk: introduce dma_readv and dma_writev

These are replacements for blk_aio_readv and blk_aio_writev that allow
customization of the data path.  They reuse the DMA helpers' DMAIOFunc
callback type, so that the same function can be used in either the
QEMUSGList or the bounce-buffered case.

This customization will be needed in the next patch to do zero-copy
SG_IO on scsi-block.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>