title: "No such device" error using pyvisa with pyvisa-py backend
body: <p>I'm trying to set up pyvisa with Python backend <code>rm=ResourceManager('@py')</code>.</p>
  
  <p>When I launch <code>rm.list_resources()</code></p>
  
  <p>I receive the following error:</p>
  
  <pre><code>libgpib: error locking board mutex!  
  Traceback (most recent call last):  
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;  
    File "/home/frank/.local/lib/python2.7/site-packages/pyvisa/highlevel.py", line 1560, in list_resources  
      return self.visalib.list_resources(self.session, query)  
    File "/home/frank/.local/lib/python2.7/site-packages/pyvisa-py/highlevel.py", line 231, in list_resources
      for key, st in sessions.Session.iter_valid_session_classes()], [])  
    File "/home/frank/.local/lib/python2.7/site-packages/pyvisa-py/gpib.py", line 57, in list_resources
      return ['GPIB0::%d::INSTR' % pad for pad in _find_listeners()]  
    File "/home/frank/.local/lib/python2.7/site-packages/pyvisa-py/gpib.py", line 35, in _find_listeners
      if gpib.listener(BOARD, i) and gpib.ask(BOARD, 1) != i:
  gpib.GpibError: listener() error: No such device (errno: 19)  
  </code></pre>
  
  <p>The output of my <code>python -m visa info</code> is:  </p>
  
  <pre><code>Machine Details:  
     Platform ID:    Linux-4.2.0-36-generic-x86_64-with-Ubuntu-15.10-wily  
     Processor:      x86_64  
  
  Python:  
     Implementation: CPython  
     Executable:     /usr/bin/python  
     Version:        2.7.10  
     Compiler:       GCC 5.2.1 20151010  
     Bits:           64bit  
     Build:          Oct 14 2015 16:09:02 (#default)  
     Unicode:        UCS4  
  
  PyVISA Version: 1.7
  
  Backends:  
     ni:  
        Version: 1.7 (bundled with PyVISA)  
        #1: /usr/lib/libvisa.so.7:  
           found by: user  
           bitness: n/a  
           Could not get more info:  
              Error while accessing /usr/lib/libvisa.so.7: /usr/lib  /libvisa.so.7: cannot open shared object file: No such file or directory
        #2: /usr/lib/x86_64-linux-gnu/libvisa.so.0.0.0:  
           found by: auto  
           bitness: 64  
           Could not get more info:  
              VI_ERROR_NSUP_ATTR (-1073807331): The specified attribute is not defined or supported by the referenced object.  
     py:  
        Version: 0.2  
        ASRL INSTR: Available via PySerial (2.7)  
        TCPIP INSTR: Available   
        USB RAW: Available via PyUSB (1.0.0rc1). Backend: libusb1  
        USB INSTR: Available via PyUSB (1.0.0rc1). Backend: libusb1  
        GPIB INSTR: Available   
        TCPIP SOCKET: Available   
     sim:  
        Version: 0.3  
        Spec version: 1.1  
  </code></pre>
  
  <p>Any idea of what I'm missing?</p>