tests/qtest: Introduce qtest_init_with_env

tests/qtest: Introduce qtest_init_with_env

Add a version of qtest_init() that takes an environment variable
containing the path of the QEMU binary. This allows tests to use more
than one QEMU binary.

If no variable is provided or the environment variable does not exist,
that is not an error. Fallback to using QTEST_QEMU_BINARY.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231018192741.25885-3-farosas@suse.de>