block/qcow2: Rename "fail_block" to just "fail"

Now alloc_refcount_block() only contains a single fail label, so it
makes more sense to just name it "fail" instead of "fail_block".

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20170613202107.10125-14-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>