tests/docker: add debian-xtensa-cross to DEBIAN_PARTIAL_IMAGES

This should have been marked when the docker recipe was added to
prevent it being used for cross compiling QEMU. Sort the
DEBIAN_PARTIAL_IMAGE list while we are at it.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>

tests/docker/Makefile.include

@@ -116,10 +116,15 @@ docker-image-travis: NOUSER=1
 docker-image-tricore-cross: docker-image-debian9
 
 # These images may be good enough for building tests but not for test builds
-DOCKER_PARTIAL_IMAGES += debian-alpha-cross debian-hppa-cross debian-m68k-cross debian-sh4-cross
-DOCKER_PARTIAL_IMAGES += debian-sparc64-cross debian-mips64-cross debian-riscv64-cross
-DOCKER_PARTIAL_IMAGES += debian-tricore-cross debian-powerpc-cross debian-ppc64-cross fedora-i386-cross
-DOCKER_PARTIAL_IMAGES += fedora-cris-cross
+DOCKER_PARTIAL_IMAGES += debian-alpha-cross
+DOCKER_PARTIAL_IMAGES += debian-hppa-cross
+DOCKER_PARTIAL_IMAGES += debian-m68k-cross debian-mips64-cross
+DOCKER_PARTIAL_IMAGES += debian-powerpc-cross debian-ppc64-cross
+DOCKER_PARTIAL_IMAGES += debian-riscv64-cross
+DOCKER_PARTIAL_IMAGES += debian-sh4-cross debian-sparc64-cross
+DOCKER_PARTIAL_IMAGES += debian-tricore-cross
+DOCKER_PARTIAL_IMAGES += debian-xtensa-cross
+DOCKER_PARTIAL_IMAGES += fedora-i386-cross fedora-cris-cross
 
 # Rules for building linux-user powered images
 #