Allow boards to use the device creation functions even if USB itself
is not available; of course the functions will fail inexorably, but
this can be okay if the calls are conditional on the existence of
some USB host controller device. This is for example the case for
hw/mips/loongson3_virt.c.
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
