add a way of getting a shell in the qemu vm

[bhcopeland]

In GitLab by @terceiro on Jun 22, 2021, 22:03 GMT+1:

This would be really useful for developers wanting to run interactive tests/fiddle around in a system running a custom kernel.

Migrated from GitLab: https://gitlab.com/Linaro/tuxrun/-/issues/19

[bhcopeland]

In GitLab by @terceiro on Jun 22, 2021, 22:05 GMT+1:

this could be implemented by calling directly the same qemu command that lava would run, and letting the user interact with it via the terminal