qemu: domain: Don't set default USB model if qemuCaps is missing
qemu: domain: Don't set default USB model if qemuCaps is missing

qemuDomainControllerDefPostParse assigns the default USB controller
model when it was not specified by the user. Skip this step if @qemuCaps
is missing so that we don't fill wrong data. This will then be fixes by
re-running the post parse callback.

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