Add Qubes RPC calls

Add two RPC calls that can run on disposable VMs:

* dz.Convert: This call simply imports the dangerzone package and runs
  the Qubes wrapper for the "document to pixels" code. This call is
  similar to the way we run the conversion part in a container.
* dz.ConvertDev: This call is for development purposes, and does the
  following:
  - First it receives the `dangerzone.conversion` module as Python
    zipfile. This way, we can quickly iterate on changes on the
    server-side part of Qubes, without altering the templates.
  - Second, it calls the Qubes wrapper for the "document to pixels"
    code, as dz.Convert does.

qubes/dz.Convert

@@ -0,0 +1,2 @@
+#!/bin/sh
+python -m dangerzone.conversion.doc_to_pixels_qubes_wrapper

qubes/dz.ConvertDev

@@ -0,0 +1,42 @@
+#!/usr/bin/env python
+
+import asyncio
+import glob
+import io
+import os
+import sys
+import tempfile
+import zipfile
+
+
+def say(msg):
+    print(msg, file=sys.stderr, flush=True)
+
+
+def main():
+    say("Debugging mode enabled")
+
+    # Get the size of the zipfile
+    size = int.from_bytes(sys.stdin.buffer.read(4))
+    say(f"Reading {size} bytes of Python zipfile")
+
+    # Read the zipfile from stdin
+    zf = sys.stdin.buffer.read(size)
+    if len(zf) < size:
+        say(f"Client closed the connection early")
+        return 1
+
+    with tempfile.NamedTemporaryFile(suffix=".zip") as t:
+        say(f"Storing the Python zipfile to {t.name}")
+        t.write(zf)
+        t.flush()
+
+        say(f"Importing the conversion module")
+        sys.path.insert(0, t.name)
+
+        from dangerzone.conversion.doc_to_pixels_qubes_wrapper import main
+        return asyncio.run(main())
+
+
+if __name__ == "__main__":
+    sys.exit(main())