qemuDomainObjBeginNestedJob: Return -2 for temporary failures
    
    If job queue is full or waiting for a job times out, the function
    returns -2 so that it can be handled in a different way by callers.
    
    The change is safe since all existing callers of
    qemuDomainObjBeginNestedJob check the return value to be less than zero.
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>