block: Introduce byte-based aio read/write

blk_aio_readv() and blk_aio_writev() are annoying in that they
can't access sub-sector granularity, and cannot pass flags.
Also, they require the caller to pass redundant information
about the size of the I/O (qiov->size in bytes must match
nb_sectors in sectors).

Add new blk_aio_preadv() and blk_aio_pwritev() functions to fix
the flaws. The next few patches will upgrade callers, then
finally delete the old interfaces.
Signed-off-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/block-backend.c

 /*
  * QEMU Block backends
  *
- * Copyright (C) 2014 Red Hat, Inc.
+ * Copyright (C) 2014-2016 Red Hat, Inc.
  *
  * Authors:
  *  Markus Armbruster <armbru@redhat.com>,
@@ -998,6 +998,14 @@ BlockAIOCB *blk_aio_readv(BlockBackend *blk, int64_t sector_num,
                         blk_aio_read_entry, 0, cb, opaque);
 }
 
+BlockAIOCB *blk_aio_preadv(BlockBackend *blk, int64_t offset,
+                           QEMUIOVector *qiov, BdrvRequestFlags flags,
+                           BlockCompletionFunc *cb, void *opaque)
+{
+    return blk_aio_prwv(blk, offset, qiov->size, qiov,
+                        blk_aio_read_entry, flags, cb, opaque);
+}
+
 BlockAIOCB *blk_aio_writev(BlockBackend *blk, int64_t sector_num,
                            QEMUIOVector *iov, int nb_sectors,
                            BlockCompletionFunc *cb, void *opaque)
@@ -1011,6 +1019,14 @@ BlockAIOCB *blk_aio_writev(BlockBackend *blk, int64_t sector_num,
                         blk_aio_write_entry, 0, cb, opaque);
 }
 
+BlockAIOCB *blk_aio_pwritev(BlockBackend *blk, int64_t offset,
+                            QEMUIOVector *qiov, BdrvRequestFlags flags,
+                            BlockCompletionFunc *cb, void *opaque)
+{
+    return blk_aio_prwv(blk, offset, qiov->size, qiov,
+                        blk_aio_write_entry, flags, cb, opaque);
+}
+
 BlockAIOCB *blk_aio_flush(BlockBackend *blk,
                           BlockCompletionFunc *cb, void *opaque)
 {

include/sysemu/block-backend.h

 /*
  * QEMU Block backends
  *
- * Copyright (C) 2014 Red Hat, Inc.
+ * Copyright (C) 2014-2016 Red Hat, Inc.
  *
  * Authors:
  *  Markus Armbruster <armbru@redhat.com>,
@@ -110,9 +110,15 @@ int64_t blk_nb_sectors(BlockBackend *blk);
 BlockAIOCB *blk_aio_readv(BlockBackend *blk, int64_t sector_num,
                           QEMUIOVector *iov, int nb_sectors,
                           BlockCompletionFunc *cb, void *opaque);
+BlockAIOCB *blk_aio_preadv(BlockBackend *blk, int64_t offset,
+                           QEMUIOVector *qiov, BdrvRequestFlags flags,
+                           BlockCompletionFunc *cb, void *opaque);
 BlockAIOCB *blk_aio_writev(BlockBackend *blk, int64_t sector_num,
                            QEMUIOVector *iov, int nb_sectors,
                            BlockCompletionFunc *cb, void *opaque);
+BlockAIOCB *blk_aio_pwritev(BlockBackend *blk, int64_t offset,
+                            QEMUIOVector *qiov, BdrvRequestFlags flags,
+                            BlockCompletionFunc *cb, void *opaque);
 BlockAIOCB *blk_aio_flush(BlockBackend *blk,
                           BlockCompletionFunc *cb, void *opaque);
 BlockAIOCB *blk_aio_discard(BlockBackend *blk,