[libvirt] gconfig: Add hostdev support v5
It's back! :) Really intend to actually get it finished and merged this time around.
Basically I mostly fixed all the minor issues in patch 3/5 (Add
GVirConfigDomainHostdevPci) pointed out in last review, including treatment of on/off
attributes (which is why the patch "libvirt-gconfig: Accept on/off as boolean
attributes" has been dropped in this version).