qemuPrepareNVRAM: Save domain conf only if domain's persistent
    
    In one of my previous patches (3a3c3780b) I've tried to fix the
    problem of nvram path disappearing on a domain that's been
    started and shut down again. I fixed this by explicitly saving
    domain's config file.  However, I did a bit of clumsy without
    realizing we have a transient domains for which we don't save the
    config file. Hence, any domain using UEFI became persistent.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>