Enhance UVM installation and configuration for Ubuntu 22.04 root filesystem

- Update README and USAGE documentation to reflect changes in rootfs handling, including the default local build from Ubuntu 22.04 packages.
- Modify the installer to support building the rootfs locally with authenticated packages and to require SHA-256 checksums for prebuilt images.
- Implement checks for setuid permissions on sudo and update the provisioning logic for guest users to ensure proper sudo access.
- Adjust tests to validate the new rootfs building process and ensure that existing functionality is preserved.
- Refactor code to improve clarity and maintainability, including the introduction of constants for Ubuntu release and guest packages.

tests/test_config.py

@@ -32,7 +32,7 @@ class ArchitectureSettingsTests(unittest.TestCase):
                     )
                     self.assertEqual(settings.architecture, architecture)
                     self.assertEqual(settings.kernel_url, f"{base}/kernels/vmlinux.bin")
-                    self.assertEqual(settings.rootfs_url, f"{base}/rootfs/bionic.rootfs.ext4")
+                    self.assertEqual(settings.rootfs_url, "")
 
     def test_preserves_individual_environment_url_overrides_on_arm64(self) -> None:
         kernel = "https://example.com/custom-arm64-kernel"
@@ -56,7 +56,7 @@ class ArchitectureSettingsTests(unittest.TestCase):
                 )
                 self.assertEqual(
                     settings.rootfs_url,
-                    overrides.get("UVM_ROOTFS_URL", f"{base}/rootfs/bionic.rootfs.ext4"),
+                    overrides.get("UVM_ROOTFS_URL", ""),
                 )
 
     def test_rejects_unsupported_hosts_even_with_custom_guest_assets(self) -> None: