qemu: remove pointless connect retry logic in agent
    
    When the agent code was first introduced back in
    
      commit c160ce3316852a797d7b06b4ee101233866e69a9
      Author: Daniel P. Berrange <berrange@redhat.com>
      Date:   Wed Oct 5 18:31:54 2011 +0100
    
        QEMU guest agent support
    
    there was code that would loop and retry the connection when opening
    the agent socket. At this time, the only thing done in between the
    opening of the monitor socket & opening of the agent socket was a
    call to set the monitor capabilities. This was a no-op on non-QMP
    versions, so in theory there could be a race which let us connect
    to the monitor while the agent socket was still not created by QEMU.
    
    In the modern world, however, we long ago mandated the use of QMP
    for managing QEMU, so we're guaranteed to have a set capabilities
    QMP call. Once we've seen a reply to this, we're guaranteed that
    QEMU has fully initialized all backends and is in its event loop.
    
    We can thus be sure the QEMU agent socket is present and don't need
    to retry connections to it, even without having the chardev FD passing
    feature.
    
    Reviewed-by: John Ferlan <jferlan@redhat.com>
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>