virtme-ng: support -- syntax to specify long exec commands

Allow to specify long commands after "--" to execute inside the
virtme-ng guest, for example:

  $ uname -r
  6.5.0-10-generic
  $ vng -- uname -r
  6.6.0-rc6-virtme

Signed-off-by: Andrea Righi <andrea.righi@canonical.com>

virtme_ng/run.py

@@ -782,6 +782,8 @@ def _get_virtme_no_virtme_ng_init(self, args):
             self.virtme_param["no_virtme_ng_init"] = ""
 
     def _get_virtme_exec(self, args):
+        if args.envs:
+            args.exec = " ".join(args.envs)
         if args.exec is not None:
             self.virtme_param["exec"] = f'--script-sh "{args.exec}"'
         else: