qcow2: Handle EAGAIN returned from update_refcount Fixes a crash during image compression Signed-off-by: Jindřich Makovička <makovick@gmail.com> Tested-by: Richard W.M. Jones <rjones@redhat.com> Reviewed-by: Max Reitz <mreitz@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>