title: pip install libvirt-python fails in virtualenv
body: <p>I currently am trying to install <code>libvirt-python</code> to a virtualenv.  </p>
  
  <p>When I run it I get this output</p>
  
  <pre><code>$ pip install libvirt-python
  Collecting libvirt-python
    Using cached libvirt-python-3.5.0.tar.gz
  Building wheels for collected packages: libvirt-python
    Running setup.py bdist_wheel for libvirt-python ... error
    Complete output from command /home/john/virtenvs/hw/bin/python3 -u -c "import setuptools, tokenize;__file__='/tmp/pip-build-3_fz8vok/libvirt-python/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" bdist_wheel -d /tmp/tmpg1j1nq_wpip-wheel- --python-tag cp35:
    Package libvirt was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libvirt.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libvirt' found
    Package libvirt was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libvirt.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libvirt' found
    running bdist_wheel
    running build
    /usr/bin/pkg-config --print-errors --atleast-version=0.9.11 libvirt
    Package libvirt was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libvirt.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libvirt' found
    error: command '/usr/bin/pkg-config' failed with exit status 1
  
    ----------------------------------------
    Failed building wheel for libvirt-python
    Running setup.py clean for libvirt-python
  Failed to build libvirt-python
  Installing collected packages: libvirt-python
    Running setup.py install for libvirt-python ... error
      Complete output from command /home/john/virtenvs/hw/bin/python3 -u -c "import setuptools, tokenize;__file__='/tmp/pip-build-3_fz8vok/libvirt-python/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /tmp/pip-leu8ei67-record/install-record.txt --single-version-externally-managed --compile --install-headers /home/john/virtenvs/hw/include/site/python3.5/libvirt-python:
      Package libvirt was not found in the pkg-config search path.
      Perhaps you should add the directory containing `libvirt.pc'
      to the PKG_CONFIG_PATH environment variable
      No package 'libvirt' found
      Package libvirt was not found in the pkg-config search path.
      Perhaps you should add the directory containing `libvirt.pc'
      to the PKG_CONFIG_PATH environment variable
      No package 'libvirt' found
      running install
      running build
      /usr/bin/pkg-config --print-errors --atleast-version=0.9.11 libvirt
      Package libvirt was not found in the pkg-config search path.
      Perhaps you should add the directory containing `libvirt.pc'
      to the PKG_CONFIG_PATH environment variable
      No package 'libvirt' found
      error: command '/usr/bin/pkg-config' failed with exit status 1
  
      ----------------------------------------
  Command "/home/john/virtenvs/hw/bin/python3 -u -c "import setuptools, tokenize;__file__='/tmp/pip-build-3_fz8vok/libvirt-python/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /tmp/pip-leu8ei67-record/install-record.txt --single-version-externally-managed --compile --install-headers /home/john/virtenvs/hw/include/site/python3.5/libvirt-python" failed with error code 1 in /tmp/pip-build-3_fz8vok/libvirt-python/
  </code></pre>
  
  <p>Without the virtualenv I'm able to import libvirt into my python shell.</p>