qemu: Resolve Coverity RESOURCE_LEAK
    
    Recent changes to the -M/--machine processing code in qemuParseCommandLine
    caused Coverity to determine there was a possible resource leak with how
    the 'list' is managed. Rather than try to add virStringFreeList calls
    everywhere - just promote list to the top of the variables and free it
    within the error processing code. Also required a couple of other tweaks
    in order to avoid double free's.