vhost-blk: Add vhost-blk support v7

vhost-blk is an in-kernel virito-blk device accelerator.

Due to lack of proper in-kernel AIO interface, this version converts
guest's I/O request to bio and use submit_bio() to submit I/O directly.
So this version any supports raw block device as guest's disk image,
e.g. /dev/sda, /dev/ram0. We can add file based image support to
vhost-blk once we have in-kernel AIO interface. There are some work in
progress for in-kernel AIO interface from Dave Kleikamp and Zach Brown:

   http://marc.info/?l=linux-fsdevel&m=133312234313122

Performance evaluation:
-----------------------------
LKVM: Fio with libaio ioengine on 1 Fusion IO device
IOPS(k)    Before    After    Improvement
seq-read   107       121      +13.0%
seq-write  130       179      +37.6%
rnd-read   102       122      +19.6%
rnd-write  125       159      +27.0%

QEMU: Fio with libaio ioengine on 1 Fusion IO device
IOPS(k)    Before    After    Improvement
seq-read   76        123      +61.8%
seq-write  139       173      +24.4%
rnd-read   73        120      +64.3%
rnd-write  75        156      +108.0%

QEMU: Fio with libaio ioengine on 1 Ramdisk device
IOPS(k)    Before    After    Improvement
seq-read   138       437      +216%
seq-write  191       436      +128%
rnd-read   137       426      +210%
rnd-write  140       415      +196%

QEMU: Fio with libaio ioengine on 8 Ramdisk device
50% read + 50% write
IOPS(k)    Before    After    Improvement
randrw     64/64     189/189  +195%/+195%

Userspace bits:
-----------------------------
1) LKVM
The latest vhost-blk userspace bits for kvm tool can be found here:
git@github.com:asias/linux-kvm.git blk.vhost-blk

2) QEMU
The latest vhost-blk userspace prototype for QEMU can be found here:
git@github.com:asias/qemu.git blk.vhost-blk

Changes in v7:
- Fix flush complete
- Add EVENTFD as dependency

Changes in v6:
- Use inline req_page_list to reduce kmalloc
- Switch to single thread model, thanks mst!
- Wait until requests fired before vhost_blk_flush to be finished

Changes in v5:
- Do not assume the buffer layout
- Fix wakeup race

Changes in v4:
- Mark req->status as userspace pointer
- Use __copy_to_user() instead of copy_to_user() in vhost_blk_set_status()
- Add if (need_resched()) schedule() in blk thread
- Kill vhost_blk_stop_vq() and move it into vhost_blk_stop()
- Use vq_err() instead of pr_warn()
- Fail un Unsupported request
- Add flush in vhost_blk_set_features()

Changes in v3:
- Sending REQ_FLUSH bio instead of vfs_fsync, thanks Christoph!
- Check file passed by user is a raw block device file

Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Asias He <asias@redhat.com>
Fix arg and argp
Drop EXPERIMENTAL
Rename vhost_blk_req_umap -> vhost_blk_req_unmap
Use put_page instead of page_cache_release
Release pages when less pages are pinned than wanted

Signed-off-by: Asias He <asias@redhat.com>