RFC virtio-blk: add multiread support

this series adds the long missing multiread support to virtio-blk.

some remarks:
- i introduced rd_merged and wr_merged block accounting stats to blockstats as a generic interface which can be set from any driver that will introduce multirequst merging in the future.
- the knob to disable request merging is not yet there. I would add it to the device properties also as a generic interface to have the same switch on for any driver that might introduce request merging in the future
- there is cleanup and iotest adjustion missing.

RFC v1->v2:
- completed Patch 1 by the example in qmp-commands.hx [Eric]
- used bool for merge in Patch 4 [Max]
- fixed a few typos in the commit msg of Patch 4 [Max]
- do not start merging and directly pass req[0]->qiov in case of num_reqs == 1
- avoid allocating memory for the multireq [Kevin]
- do not import block_int.h and add appropiate iface to block-backend [Kevin]
- removed debug output and added trace event for multireq [Kevin]
- fixed alloc hint for the merge qiov [Kevin]
- currently did not split virtio_submit_multireq into rw code since the redundant code would now be much bigger part than in the original patch.
- added a merge_qiov to VirtioBlockRequest. Abusing the qiov was not possible because it is initialized externally with guest memory [Kevin]
- added a pointer to VirtioBlockRequest to create a linked list of VirtioBlockBlockRequests. This list is used to complete all requests belonging to a multireq [Kevin]

Peter Lieven (4):
block: add accounting for merged requests hw/virtio-blk: add a constant for max number of merged requests
block-backend: expose bs->bl.max_transfer_length
virtio-blk: introduce multiread

 block.c                         |    2 +
 block/accounting.c              |    7 ++
 block/block-backend.c           |    5 +
 block/qapi.c                    |    2 +
 hmp.c                           |    6 +-
 hw/block/dataplane/virtio-blk.c |   10 +-
 hw/block/virtio-blk.c           |  219 ++++++++++++++++++++++-----------------
 include/block/accounting.h      |    3 +
 include/hw/virtio/virtio-blk.h  |   23 ++--
 include/sysemu/block-backend.h  |    1 +
 qapi/block-core.json            |    9 +-
 qmp-commands.hx                 |   22 +++-
 trace-events                    |    1 +
 13 files changed, 197 insertions(+), 113 deletions(-)

1.7.9.5