Since the ahci-test uses qemu-img, add a dependency to build it
before using it.
This fixes:

  $ gmake check-qtest V=1
  QTEST_QEMU_BINARY=x86_64-softmmu/qemu-system-x86_64 QTEST_QEMU_IMG=qemu-img tests/ahci-test
  Failed to execute child process "/tmp/qemu-test.19tMRF/qemu-img" (No such file or directory)
  ERROR:tests/libqos/libqos.c:192:mkimg: assertion failed: (ret && !err)

Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
Slighly related is when vm-tests expect qemu-img available:
https://lists.gnu.org/archive/html/qemu-devel/2018-06/msg08415.html

  $ make vm-build-ubuntu.i386
  Traceback (most recent call last):
    File "source/qemu/tests/vm/basevm.py", line 236, in main
      return vm.build_image(args.image)
    File "tests/vm/ubuntu.i386", line 67, in build_image
      subprocess.check_call(["qemu-img", "resize", img_tmp, "50G"])
  OSError: [Errno 2] No such file or directory
  tests/vm/Makefile.include:23: recipe for target 'tests/vm/ubuntu.i386.img' failed
  make: *** [tests/vm/ubuntu.i386.img] Error 2

A better fix would be checking those tools via ./configure...
---
 tests/Makefile.include | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/Makefile.include b/tests/Makefile.include
index 19b4c0a696..5e03416c81 100644
--- a/tests/Makefile.include
+++ b/tests/Makefile.include
@@ -706,7 +706,7 @@ tests/prom-env-test$(EXESUF): tests/prom-env-test.o $(libqos-obj-y)
 tests/rtas-test$(EXESUF): tests/rtas-test.o $(libqos-spapr-obj-y)
 tests/fdc-test$(EXESUF): tests/fdc-test.o
 tests/ide-test$(EXESUF): tests/ide-test.o $(libqos-pc-obj-y)
-tests/ahci-test$(EXESUF): tests/ahci-test.o $(libqos-pc-obj-y)
+tests/ahci-test$(EXESUF): tests/ahci-test.o $(libqos-pc-obj-y) qemu-img$(EXESUF)
 tests/ipmi-kcs-test$(EXESUF): tests/ipmi-kcs-test.o
 tests/ipmi-bt-test$(EXESUF): tests/ipmi-bt-test.o
 tests/hd-geo-test$(EXESUF): tests/hd-geo-test.o
-- 
2.20.1