block/vdi: Add locking for parallel requests

block/vdi: Add locking for parallel requests

When allocating a new cluster, the first write to it must be the one
doing the allocation, because that one pads its write request to the
cluster size; if another write to that cluster is executed before it,
that write will be overwritten due to the padding.

See https://bugs.launchpad.net/qemu/+bug/1422307 for what can go wrong
without this patch.

Cc: qemu-stable <qemu-stable@nongnu.org>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>