Report an error if registering an event loop twice
Report an error if registering an event loop twice

The C library will now ignore an attempt to register an event
loop twice. It is unable to report an error in this case though
due to the C API returning 'void'. To improve this we must
manually report an error at the python level.

Signed-off-by: Daniel P. Berrange <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

libvirt/libvirt-python,libvirt/libvirt-python,libvirt/libvirt-python