conf: Introduce isolation groups

conf: Introduce isolation groups

Isolation groups will eventually allow us to make sure certain
devices, eg. PCI hostdevs, are assigned to guest PCI buses in
a way that guarantees improved isolation, error detection and
recovery for machine types and hypervisors that support it,
eg. pSeries guest on QEMU.

This patch merely defines storage for the new information
we're going to need later on and makes sure it is passed from
the hypervisor driver (QEMU / bhyve) down to the generic PCI
address allocation code.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Laine Stump <laine@laine.org>