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.

uvm/config.py

@@ -31,7 +31,7 @@ class Settings:
     default_vcpu: float = 1
     default_ssh_user: str = "root"
     kernel_url: str = ""
-    rootfs_url: str = ""
+    rootfs_url: str = ""  # Empty selects the locally built, package-managed Ubuntu image.
     firecracker_sha256: str | None = None
     firecracker_binary_sha256: str | None = None
     kernel_sha256: str | None = None
@@ -56,8 +56,6 @@ class Settings:
         asset_base = f"{DEFAULT_GUEST_ASSETS_URL}/{architecture}"
         if not self.kernel_url:
             object.__setattr__(self, "kernel_url", f"{asset_base}/kernels/vmlinux.bin")
-        if not self.rootfs_url:
-            object.__setattr__(self, "rootfs_url", f"{asset_base}/rootfs/bionic.rootfs.ext4")
 
         if self.gateway not in self.network:
             raise ConfigurationError(