qemu: Use qemu-xhci with no fallback on loongarch64

qemu: Use qemu-xhci with no fallback on loongarch64

The architecture was introduced at a time when USB3 in
general, and qemu-xhci in particular, had already been
well established for years. Having USB1 controllers as a
fallback was something that happened by mistake due to
the way the pre-existing code was organized rather than
because of a conscious decision. Make things work the
way they should have in the first place.

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>