hw/usb/hcd-xhci-pci: Use event ring 0 if mapping unsupported

hw/usb/hcd-xhci-pci: Use event ring 0 if mapping unsupported

The XHCI specification, section 4.17.1 specifies that "If the
Number of Interrupters (MaxIntrs) field is greater than 1, then
Interrupter Mapping shall be supported." and "If Interrupter
Mapping is not supported, the Interrupter Target field shall be
ignored by the xHC and all Events targeted at Interrupter 0."

QEMU's XHCI device has so far not specially addressed this case,
so we add a check to xhci_event() to redirect to event ring and
interrupt 0 if mapping is disabled.

Signed-off-by: Phil Dennis-Jordan <phil@philjordan.eu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20241227121336.25838-4-phil@philjordan.eu>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>