qcow2: Merge the writing of the COW regions with the guest data

If the guest tries to write data that results on the allocation of a
new cluster, instead of writing the guest data first and then the data
from the COW regions, write everything together using one single I/O
operation.

This can improve the write performance by 25% or more, depending on
several factors such as the media type, the cluster size and the I/O
request size.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qcow2.h

@@ -343,6 +343,13 @@ typedef struct QCowL2Meta
      */
     Qcow2COWRegion cow_end;
 
+    /**
+     * The I/O vector with the data from the actual guest write request.
+     * If non-NULL, this is meant to be merged together with the data
+     * from @cow_start and @cow_end into one single write operation.
+     */
+    QEMUIOVector *data_qiov;
+
     /** Pointer to next L2Meta of the same write request */
     struct QCowL2Meta *next;