pseries: Remove "busname" property for PCI host bridge

Currently the "spapr-pci-host-bridge" device has a "busname" property which
can be used to override the default assignment of qbus names for the bus
subordinate to the PHB.  We use that for the default primary PCI bus, to
make libvirt happy, which expects there to be a bus named simply "pci".
The default qdev core logic would name the bus "pci.0", and the pseries
code would otherwise name it "pci@800000020000000" which is the name it
is given in the device tree based on its BUID.

The "busname" property is rather clunky though, so this patch simplifies
things by just using a special case hack for the default PHB, setting
busname to "pci" when index=0.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
