Colon in user results in invalid qrexec command

[DemiMarie]

How to file a helpful issue

Qubes OS release

R4.2, but this problem is probably older than R4.1.

Brief summary

If one passes --user=a:b (or any other string containing a colon) to qvm-run in dom0, the colon is interpreted as the end of the username, rather than as part of it.

Steps to reproduce

Either:

1. Pass a username to qvm-run --user= that contains a colon.
2. Include a user= keyword in a qrexec policy that contains a colon.

Expected behavior

Error stating that usernames in qrexec cannot contain a colon.

Actual behavior

Colon is interpreted as end of the username, and everything after it is interpreted as the command. For instance, qvm-run --user=DEFAULT:nogui is equivalent to qvm-run --no-gui.