Disable libvirtd by default when building on Win32
    
    We don't support building libvirtd on Win32 since we lack the
    fork/exec feature needed for the stateful drivers. Disable this
    by default, so users can just do 'mingw32-configure' with no
    special args required.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>