doc/demo: Fix and suppress ShellCheck warnings

ShellCheck reports (SC2034) that __qemu_arch is not used. Use it,
and silence the resulting SC2086 warning as we want word splitting on
options we pass with it.

While at it, silence SC2317 warnings for commands in cleanup() that
appear to be unreachable: cleanup() is only called as trap.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

doc/demo.sh

@@ -110,6 +110,7 @@ next() {
 }
 
 # cleanup() - Terminate pasta and passt, clean up, restore TTY settings
+# shellcheck disable=SC2317
 cleanup() {
 	[ -f "${DEMO_DIR}/pasta.pid" ] && kill "$(cat "${DEMO_DIR}/pasta.pid")"
 	[ -f "${DEMO_DIR}/passt.pid" ] && kill "$(cat "${DEMO_DIR}/passt.pid")"
@@ -223,7 +224,8 @@ into_ns() {
 	echo "Use ^C to terminate it."
 	next
 
-	cmd qrap 5 qemu-system-x86_64 -M pc,accel=kvm:tcg		    \
+	# shellcheck disable=SC2086
+	cmd qrap 5 ${__qemu_arch}					    \
 		-smp "$(nproc)" -m 1024					    \
 		-nographic -serial stdio -nodefaults -no-reboot -vga none   \
 		-initrd "${DEMO_DIR}/demo.img"				    \