conf: put all guest-related HostdevDef data in one object

conf: put all guest-related HostdevDef data in one object

To help consolidate the commonality between virDomainHostdevDef and
virDomainNetDef into as few members as possible (and because I
think it makes sense), this patch moves the rombar and bootIndex
members into the "info" member that is common to both (and to all the
other structs that use them).

It's a bit problematic that this gives rombar and bootIndex to many
device types that don't use them, but this is already the case for the
master and mastertype members of virDomainDeviceInfo, and is properly
commented as such in the definition.

Note that this opens the door to supporting rombar for other devices
that are attached to the guest PCI bus - virtio-blk-pci,
virtio-net-pci, various other network adapters - which which have that
capability in qemu, but previously had no support in libvirt.