preallocate() will be locked. This is required because
qcow2_alloc_cluster_link_l2() assumes that it runs under a lock that it
can drop while COW is being performed.
Signed-off-by: NZhi Yong Wu <wuzhy@linux.vnet.ibm.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>