qemu: delete rule target on error

Instruct make to remove any rule target on error. This prevetns
situation where there was an error during build but generated file still
stays behind.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

rules.mak

@@ -47,3 +47,6 @@ cc-option = $(if $(shell $(CC) $1 $2 -S -o /dev/null -xc /dev/null \
 %.h-timestamp: %.mak
 	$(call quiet-command, sh $(SRC_PATH)/create_config < $< > $@, "  GEN   $*.h")
 	@cmp $@ $*.h >/dev/null 2>&1 || cp $@ $*.h
+
+# will delete the target of a rule if commands exit with a nonzero exit status
+.DELETE_ON_ERROR: