storage: Rework ret logic in storageBackendUpdateVolTargetInfo

storage: Rework ret logic in storageBackendUpdateVolTargetInfo

Rather than overload @ret with trying serve multiple purposes,
let's initialize @ret to -1 and introduce an @rc function return
value that can be used for functions that may return -1 or -2
and only override @ret when rc < 0.

Signed-off-by: John Ferlan <jferlan@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>