block: convert qcow2, qcow2, and vmdk to .bdrv_co_is_allocated()

The qcow2, qcow, and vmdk block drivers are based on coroutines.  They have a
coroutine mutex which protects internal state.  We can convert the
.bdrv_is_allocated() function to .bdrv_co_is_allocated() by holding the mutex
around the cluster lookup operation.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
