virTimeBackOffWait: Avoid long periods of sleep
    
    While connecting to qemu monitor, the first thing we do is wait
    for it to show up. However, we are doing it with some timeout to
    avoid indefinite waits (e.g. when qemu doesn't create the monitor
    socket at all). After beaa447a29 we are using exponential back
    off timeout meaning, after the first connection attempt we wait
    1ms, then 2ms, then 4 and so on.  This allows us to bring down
    wait time for small domains where qemu initializes quickly.
    However, on the other end of this scale are some domains with
    huge amounts of guest memory. Now imagine that we've gotten up to
    wait time of 15 seconds. The next one is going to be 30 seconds,
    and the one after that whole minute. Well, okay - with current
    code we are not going to wait longer than 30 seconds in total,
    but this is going to change in the next commit.
    
    The exponential back off is usable only for first few iterations.
    Then it needs to be caped (one second was chosen as the limit)
    and switch to constant wait time.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>