Added #ifdef for VIR_DOMAIN_PMSUSPENDED

I'm not sure of the exact version that it showed up in libvirt, but the VIR_DOMAIN_PMSUSPENDED constant was made available back in January (http://libvirt.org/git/?p=libvirt.git;a=commit;h=e0642059367436c27bc031df3b03ccdc1d818350) in the libvirt library. To maintain backward compatibility for older versions of libvirt, the node constant is wrapped in #ifdef so as not to cause compilation problems.