vmtest: Set the default start timeout to 1800s

This code is imported from unknown places, some setting the timeout and
others not setting it. This has resulted in playing whack-a-mole with
the timeout override so let's set it long and if a user wants a shorter
timeout they can override it.

Author: brlane-rht

test/scripts/vmtest/vm.py

@@ -255,7 +255,7 @@ def _gen_qemu_cmdline(self, snapshot, use_ovmf):
         return qemu_cmdline
 
     # XXX: move args to init() so that __enter__ can use them?
-    def start(self, wait_event="ssh", snapshot=True, use_ovmf=False, timeout_sec=120):
+    def start(self, wait_event="ssh", snapshot=True, use_ovmf=False, timeout_sec=1800):
         if self.running():
             return
         self._ssh_port = get_free_port()