qemu: Validate PCI support for USB controllers

qemu: Validate PCI support for USB controllers

Attempting to use a USB controller that's a PCI device with
a machine type that doesn't support PCI should result in an
error.

Note that, while all USB controllers supported by the libvirt
QEMU driver today are PCI devices, QEMU itself implements
machine types that come with non-PCI USB controllers. Having
a separate helper with a switch/case statement ensures that
things will need to be updated accordingly if libvirt will
ever grow support for those USB controllers.

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>