cloud-init: fix missing pyserial dependency

and check for all required imports as per requirements.txt

While debugging missing metadata in the cloud-init boot local phase
in a NixOS VM on SmartOS, the following error was observed:
SystemError: Unable to open /dev/ttyS1
Adding the missing pyserial dependency fixes the above error and
cloud-init metadata detection now works in NixOS VMs on SmartOS

pkgs/tools/virtualization/cloud-init/default.nix

@@ -51,6 +51,7 @@ python3.pkgs.buildPythonApplication rec {
     jsonschema
     netifaces
     oauthlib
+    pyserial
     pyyaml
     requests
   ];