Silicon/Qemu: register non-discoverable XHCI for SbsaQemu

The EHCI component was never mapped in edk2, possibly because it was
unusable on systems with no memory below 4GB. Now we have an XHCI
component instead, register it properly as a non-dicoverable device.

Signed-off-by: Yuquan Wang <wangyuquan1236@phytium.com.cn>
[Broke this out as separate patch.]
Signed-off-by: Leif Lindholm <quic_llindhol@quicinc.com>
Tested-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>