qemu: avoid rare race when undefining domain
    
    When one domain is being undefined and at the same time started, for
    example, there is a possibility of a rare problem occuring.
    
     - Thread 1 does virDomainUndefine(), has the lock, checks that the
       domain is active and because it's not, calls
       virDomainObjListRemove().
    
     - Thread 2 does virDomainCreate() and tries to lock the domain.
    
     - Thread 1 needs to lock domain list in order to remove the domain from
       it, but must unlock domain first (proper order is to lock domain list
       first and the domain itself second).
    
     - Thread 2 grabs the lock, starts the domain and releases the lock.
    
     - Thread 1 grabs the lock and removes the domain from list.
    
    With this patch:
    
     - qemuDomainRemoveInactive() creates a QEMU_JOB_MODIFY if that's
       possible, but since it must remove the domain from list either way,
       it continues even when starting the job failed.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1150505
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>