storage: On error unlink created file in virFileOpen{As|Forked}

storage: On error unlink created file in virFileOpen{As|Forked}

After a successful creation of a file, if some other call results
in returning a failure, let's unlink the file we created to prevent
another round trip or confusion in the caller. In particular, this
function can be called during a storage backend buildVol, so in order
to ensure that caller doesn't need to distinguish between failed create
or some other failure after create, just remove the volume we created.

Signed-off-by: John Ferlan <jferlan@redhat.com>