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>