tests/vm: Introduce get_qemu_packages_from_lcitool_json() helper

Add the get_qemu_packages_from_lcitool_json() helper which return
such package list from a lcitool env var file in JSON format.

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230711144922.67491-4-philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>

tests/vm/basevm.py

@@ -27,6 +27,7 @@
 import multiprocessing
 import traceback
 import shlex
+import json
 
 from qemu.machine import QEMUMachine
 from qemu.utils import get_info_usernet_hostfwd_port, kvm_available
@@ -501,6 +502,16 @@ def gen_cloud_init_iso(self):
                               stderr=self._stdout)
         return os.path.join(cidir, "cloud-init.iso")
 
+    def get_qemu_packages_from_lcitool_json(self, json_path=None):
+        """Parse a lcitool variables json file and return the PKGS list."""
+        if json_path is None:
+            json_path = os.path.join(
+                os.path.dirname(__file__), "generated", self.name + ".json"
+            )
+        with open(json_path, "r") as fh:
+            return json.load(fh)["pkgs"]
+
+
 def get_qemu_path(arch, build_path=None):
     """Fetch the path to the qemu binary."""
     # If QEMU environment variable set, it takes precedence