doc: update the GPU passthrough HPC feature entry
doc: update the GPU passthrough HPC feature entry

This updates some busted links, adds ironic and powervm to the
table header, and copies a description from the admin guide.

Change-Id: If146a26a8d0c66a3ff218c62624e3a130744dde5

mahak/nova,mahak/nova,gooddata/openstack-nova,mahak/nova,gooddata/openstack-nova,phenoxim/nova,rahulunair/nova,rahulunair/nova,klmitch/nova,phenoxim/nova,klmitch/nova,gooddata/openstack-nova,openstack/nova,openstack/nova,mikalstill/nova,klmitch/nova,gooddata/openstack-nova,mikalstill/nova,rahulunair/nova,openstack/nova,klmitch/nova,mikalstill/nova