leave out the default USB controller only on i440fx during migration

Further followup discussions in list on commit 192a53e0 concluded
that we should be leaving out the USB controller only for
i440fx machines as default USB can be used by someone on q35
at random slots.
Signed-off-by: NShivaprasad G Bhat <sbhat@linux.vnet.ibm.com>

src/qemu/qemu_domain.c

@@ -2932,7 +2932,8 @@ qemuDomainDefFormatBuf(virQEMUDriverPtr driver,
          *  with libvirt <= 0.9.4. Limitation doesn't apply to other archs
          *  and can cause problems on PPC64.
          */
-        if (ARCH_IS_X86(def->os.arch) && usb && usb->idx == 0 && usb->model == -1) {
+        if (ARCH_IS_X86(def->os.arch) && qemuDomainMachineIsI440FX(def) &&
+            usb && usb->idx == 0 && usb->model == -1) {
             VIR_DEBUG("Removing default USB controller from domain '%s'"
                       " for migration compatibility", def->name);
             toremove++;