tests/libqtest: Use a proper error message if QTEST_QEMU_BINARY is missing

The user can currently still cause an abort() if running certain tests
(like the prom-env-test) without setting the QTEST_QEMU_BINARY first.
A similar problem has been fixed with commit 7c933ad6
already, but forgot to also take care of the qtest_get_arch() function,
so let's introduce a proper wrapper around getenv("QTEST_QEMU_BINARY")
that can be used in both locations now.

Buglink: https://bugs.launchpad.net/qemu/+bug/1713434


Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>