qemu: implement <target chassisNr='n'/> subelement/attribute of <controller>

qemu: implement <target chassisNr='n'/> subelement/attribute of <controller>

This uses the new subelement/attribute in two ways:

1) If a "pci-bridge" pci controller has no chassisNr attribute, it
will automatically be set to the controller's index as soon as the
controller's PCI address is known (during
qemuDomainAssignPCIAddresses()).

2) when creating the commandline for a pci-bridge device, chassisNr
will be used to set qemu's chassis_nr option (rather than the previous
practice of hard-coding it to the controller's index).