Support uuid syntax

[ben-grande]

After the following PR is merged:

• Implement UUID support in qrexec QubesOS/qubes-core-qrexec#135

Add support for uuid: syntax. It can be specified in the source, destination, target_uuid=uuid: and can be of @dispvm:uuid: in the previous fields.

The UUID is generated from Python's uuid library, as seen in qubes-core-admin/qubes/vm/init.py respects RFC 4122.

It is a 32 chars string without couting the - hyphens (+4=36). Composed with of 00000000-0000-0000-0000-000000000000 (8+4+4+4+12), being hexadecimal strings, [0-9a-fA-F]. Note that uppercase letters depends on the response of QubesOS/qubes-core-qrexec#135 (comment).

[ben-grande]

I asked for strict validation of the UUID syntax, I don't see if it blocked upper-case letters, although it would be easier at least with python on the parser to use .lower() before evaluation.

As lower-case was preferred, I am enforcing usage of lower-case on the policy.