spapr_pci: QOM'ify sPAPR PCI host bridge

spapr_pci: QOM'ify sPAPR PCI host bridge

Introduce type constant. Introduce cast macro to drop bogus busdev field
that would've broken SYS_BUS_DEVICE(). Avoid accessing parent fields
directly.

Free the identifier phb as acronym for PCI_HOST_BRIDGE.

Updated against conflicting merge from branch 'agraf/ppc-for-upstream'
(0d16fdd732d1b211842fa96b7c90ddf9e6bde0e4), which removed busdev field
differently, moved some code around and added new occurrences of 'phb'.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>