qemu: turn qemuDomainPCIAddressBus into a struct

qemu: turn qemuDomainPCIAddressBus into a struct

qemuDomainPCIAddressBus was an array of QEMU_PCI_ADDRESS_SLOT_LAST
uint8_t's, which worked fine as long as every PCI bus was
identical. In the future, some PCI busses will allow connecting PCI
devices, and some will allow PCIe devices; also some will only allow
connection of a single device, while others will allow connecting 31
devices.

In order to keep track of that information for each bus, we need to
turn qemuDomainPCIAddressBus into a struct, for now with just one
member:

   uint8_t slots[QEMU_PCI_ADDRESS_SLOT_LAST];

Additional members will come in later patches.

The item in qemuDomainPCIAddresSet that contains the array of
qemuDomainPCIAddressBus is now called "buses" to be more consistent
with the already existing "nbuses" (and with the new "slots" array).