storage: Error out earlier if the volume target path already exists

storage: Error out earlier if the volume target path already exists

https://bugzilla.redhat.com/show_bug.cgi?id=832302

It's odd to fall through to buildVol, and the existed file is
removed when buildVol fails. This checks if the volume target
path already exists in createVol. The reason for not using
error like "Volume already exists" is that there isn't volume
maintained by libvirt for the path until a operation like
pool-refresh, using error like that will just cause confusion.