4513872 virsh: report errors for empty strings

Authored and Committed by Ján Tomko 8 years ago
    virsh: report errors for empty strings
    
    Several callers were using vshCommandOptString without setting an error.
    Use vshCommandOptStringReq which sets the error.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1281707
    
        
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