*spapr_pci: fix unplug for boot-time devices

This enables unplug for PCI devices present during initial boot,
which currently triggers an assert due to a false assumption that
all unpluggable devices must have been previously hotplugged.

To support this we also need to re-work DRC table initialization to
occur early during machine creation, namely by allocating the top-level
table prior to the creation of any PHBs (during spapr machine init),
and by moving drc entry initialization for PHBs into the init/realize
callback.

We also need to set the initial isolation state of slots manually,
since this generally only gets done via guest tools during hotplug.

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
(cherry picked from commit b65a9303339ce1b9572320f19db5585a41cf88a5)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

Conflicts:
	hw/ppc/spapr_pci.c

* fixed up forward-port issues.
* this should be integrated into prior patches

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>