ahci-test: Add dependency to qemu-img tool

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: NJohn Snow <jsnow@redhat.com>
Signed-off-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

tests/Makefile.include

@@ -758,7 +758,7 @@ 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