qemu: Set alias for memory cell in qemuBuildMemoryCellBackendStr
    
    Very soon qemuBuildMemoryBackendStr() is going to use memory cell
    aliases. Therefore set one. At the same time, move it a bit
    further - if virAsprintf() fails, there's no point in setting
    rest of the members.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
    Reviewed-by: John Ferlan <jferlan@redhat.com>