qemu: support new pci controller model "pcie-expander-bus"

qemu: support new pci controller model "pcie-expander-bus"

This is backed by the qemu device pxb-pcie, which will be available in
qemu 2.6.0.

As with pci-expander-bus (which uses qemu's pxb device), the busNr
attribute and <node> subelement of <target> are used to set the bus_nr
and numa_node options.

During post-parse we validate that the domain's machinetype is
q35-based (since the device shows up for 440fx-based machinetypes, but
is unusable), as well as checking that <node> specifies a node that is
actually configured on the guest.