tests: add new OSBUILD_TEST_QEMU_GUI env to make debugging easier

When running tests locally it can be useful to have a full qemu
gui. E.g. the install test is not putting anything on the serial
port AFAICT. So the new environment `OSBUILD_TEST_QEMU_GUI=1` will
bring up a graphical qemu during the tests.

Use as e.g.
```
$ sudo OSBUILD_TEST_QEMU_GUI=1 pytest -s -vv -rs './test/test_build.py::test_iso_installs[iso]'
```

test/vm.py

@@ -120,13 +120,14 @@ def start(self, wait_event="ssh", snapshot=True, use_ovmf=False):
             "-m", self.MEM,
             # get "illegal instruction" inside the VM otherwise
             "-cpu", "host",
-            "-nographic",
             "-serial", "stdio",
             "-monitor", "none",
             "-netdev", f"user,id=net.0,hostfwd=tcp::{self._ssh_port}-:22",
             "-device", "rtl8139,netdev=net.0",
             "-qmp", f"unix:{self._qmp_socket},server,nowait",
         ]
+        if not os.environ.get("OSBUILD_TEST_QEMU_GUI"):
+            qemu_cmdline.append("-nographic")
         if use_ovmf:
             qemu_cmdline.extend(["-bios", find_ovmf()])
         if self._cdrom: