machine-xilinx-qemu: Refer to the standard helper then use PREFERRED_PROVIDER
IMAGE_CLASSES += "qemuboot-xilinx"
# depend on qemu-helper-native, which will depend on QEMU
-EXTRA_IMAGEDEPENDS += "qemu-xilinx-helper-native"
+EXTRA_IMAGEDEPENDS += "qemu-helper-native"
PREFERRED_PROVIDER_qemu-helper-native = "qemu-xilinx-helper-native"
PREFERRED_PROVIDER_qemu = "qemu-xilinx"
