qcow2: flush refcount cache correctly in alloc_refcount_block()

qcow2: flush refcount cache correctly in alloc_refcount_block()

update_refcount() affects the refcount cache, it does not write to disk.
Therefore bdrv_flush(bs->file) does nothing.  We need to flush the
refcount cache in order to write out the refcount updates!

While we're here also add error returns when qcow2_cache_flush() fails.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 9991923b262dc35f6dd8393ab4853edd7fc3724f)

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