[21.05] fc.qemu: update to reduce test flakiness

[ctheune]

Re PL-133015

@flyingcircusio/release-managers

Release process

Impact:

Changelog:

PR release workflow (internal)

• PR has internal ticket
• internal issue ID (PL-…) part of branch name
• internal issue ID mentioned in PR description text
• ticket is on Platform agile board
• ticket state set to Pull request ready
• if ticket is more urgent than within the next few days, directly contact a member of the Platform team

Design notes

• Provide a feature toggle if the change might need to be adjusted/reverted quickly depending on context. Consider whe
  ther the default should be on or off. Example: rate limiting.

not needed

• All customer-facing features and (NixOS) options need to be discoverable from documentation. Add or update relevant documentation such that hosted and guided customers can understand it as well.

not customer facing

Security implications

• Security requirements defined? (WHERE)

reliability of our workflows to ensure ability to act

• Security requirements tested? (EVIDENCE)

tests work, but flakiness will be proven over time