tests: add missing dependency to build QTEST_QEMU_BINARY

This allow a one liner from fresh repository clone, i.e.:

  ./configure && make -j check-qtest-aarch64

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>