virtio-blk: simplify multiwrite calling conventions

Pass the MultiReqBuffer structure down all the way to the I/O submission
instead of takin it apart.  Also mark num_writes unsigned as it can't
go negative, and take the check for any pending I/O requests into the
submission function.  Last but not least rename do_multiwrite to
virtio_submit_multiwrite to fit the general naming scheme and make clear
what it does.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
