Add QEMU driver sidecar-mode tests and README updates

[mangelajo]

Summary

PR #910 added a significant sidecar execution path to the QEMU driver (launcher_socket, _wrap_command, socket-based serial/console), but driver unit tests and README documentation for that mode were deferred.

Background

From PR #910:

Driver test and README updates for sidecar mode will be tracked in a follow-up PR — this PR is already quite large.

Original review note: missing test/README updates.

What needs to be done

1. Add unit tests in jumpstarter_driver_qemu/driver_test.py covering:
   • _wrap_command / launcher socket mode
   • _work_dir / shared-volume path branching
   • serial/console behavior when running via jumpstarter-exec
2. Update jumpstarter-driver-qemu README with sidecar-mode configuration (launcher_socket, shared volume paths, ExporterSet usage)
3. Prefer e2e-style coverage where practical (server + client), mock only when system tooling makes that unreliable

Acceptance criteria

• Sidecar-mode code paths covered by package tests (make pkg-test-jumpstarter-driver-qemu)
• README documents sidecar / ExporterSet usage
• Docs symlink under docs/source/reference/package-apis/drivers/ stays accurate if README changes

Tracking

• Jira: https://redhat.atlassian.net/browse/PITCREW-502
• PR: feat: QEMU ExporterSet sidecar pattern with jumpstarter-exec (PITCREW-487) #910
• Package: python/packages/jumpstarter-driver-qemu/