leave out the default USB controller only on i440fx during migration
leave out the default USB controller only on i440fx during migration

Further followup discussions in list on commit 192a53e 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: Shivaprasad G Bhat <90f736271cc61454bf7410f17f3d154578105aa0@linux.vnet.ibm.com>

zippy2/libvirt,eskultety/libvirt,zippy2/libvirt,datto/libvirt,jfehlig/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,libvirt/libvirt,fabianfreyer/libvirt,VenkatDatta/libvirt,nertpinx/libvirt,nertpinx/libvirt,eskultety/libvirt,taget/libvirt,olafhering/libvirt,datto/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,jardasgit/libvirt,olafhering/libvirt,andreabolognani/libvirt,jardasgit/libvirt,jfehlig/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,crobinso/libvirt,taget/libvirt,taget/libvirt,eskultety/libvirt,olafhering/libvirt,taget/libvirt,nertpinx/libvirt,jfehlig/libvirt,zippy2/libvirt,crobinso/libvirt,crobinso/libvirt,datto/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,jardasgit/libvirt,crobinso/libvirt,datto/libvirt,taget/libvirt,andreabolognani/libvirt,libvirt/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,libvirt/libvirt,eskultety/libvirt,fabianfreyer/libvirt,libvirt/libvirt,olafhering/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,zippy2/libvirt,datto/libvirt,eskultety/libvirt,jfehlig/libvirt