qemu: Fix PCI/USB handling for Arm realview boards

qemu: Fix PCI/USB handling for Arm realview boards

Only the -eb variants of the realview board support PCI
devices, so those are the only ones that should automatically
get a USB controller (addDefaultUSB). libvirt will currently
add one for the other realview variants too, but that will
result in QEMU reporting an error due to lack of PCI support
as soon as the domain is started.

Additionally, they should get a PCI controller added
automatically (addPCIRoot) too, same as versatilepb.

Finally, qemuDomainSupportsPCI() should correctly report the
fact that these machine types support PCI.

As a consequence of these fixes, the USB controllers now
correctly get assigned PCI addresses across the board.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>