qcow2: Metadata preallocation

This introduces a qemu-img create option for qcow2 which allows the metadata to
be preallocated, i.e. clusters are reserved in the refcount table and L1/L2
tables, but no data is written to them. Metadata is quite small, so this
happens in almost no time.

Especially with qcow2 on virtio this helps to gain a bit of performance during
the initial writes. However, as soon as create a snapshot, we're back to the
normal slow speed, obviously. So this isn't the real fix, but kind of a cheat
while we're still having trouble with qcow2 on virtio.

Note that the option is disabled by default and needs to be specified
explicitly using qemu-img create -f qcow2 -o preallocation=metadata.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

block_int.h

@@ -37,6 +37,7 @@
 #define BLOCK_OPT_BACKING_FILE  "backing_file"
 #define BLOCK_OPT_BACKING_FMT   "backing_fmt"
 #define BLOCK_OPT_CLUSTER_SIZE  "cluster_size"
+#define BLOCK_OPT_PREALLOC      "preallocation"
 
 typedef struct AIOPool {
     void (*cancel)(BlockDriverAIOCB *acb);