qcow2: flush refcount cache correctly in qcow2_write_snapshots()

qcow2: flush refcount cache correctly in qcow2_write_snapshots()

Since qcow2 metadata is cached we need to flush the caches, not just the
underlying file.  Use bdrv_flush(bs) instead of bdrv_flush(bs->file).

Also add the error return path when bdrv_flush() fails and move the
flush after checking for qcow2_alloc_clusters() failure so that the
qcow2_alloc_clusters() error return value takes precedence.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit f6977f15561973d4a67b6aa46da88aa678c505dd)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>