Fix race condition reconnecting to vms & loading configs
    
    The following sequence
    
     1. Define a persistent QMEU guest
     2. Start the QEMU guest
     3. Stop libvirtd
     4. Kill the QEMU process
     5. Start libvirtd
     6. List persistent guests
    
    At the last step, the previously running persistent guest
    will be missing. This is because of a race condition in the
    QEMU driver startup code. It does
    
     1. Load all VM state files
     2. Spawn thread to reconnect to each VM
     3. Load all VM config files
    
    Only at the end of step 3, does the 'virDomainObjPtr' get
    marked as "persistent". There is therefore a window where
    the thread reconnecting to the VM will remove the persistent
    VM from the list.
    
    The easy fix is to simply switch the order of steps 2 & 3.
    
    In addition to this though, we must only attempt to reconnect
    to a VM which had a non-zero PID loaded from its state file.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>