qemu: Use qemuGetCompressionProgram for error paths
    
    Let's do some more code reuse - there are 3 other callers that care to
    check/get the compress program. Each of those though cares whether the
    requested cfg image is valid and exists. So, add a parameter to handle
    those cases.
    
    NB: We won't need to initialize the returned value in the case where
    the cfg image doesn't exist since the called program will handle that.