os: document qemu-booting custom builds

[Bubblemelon]

I've added some facts that I found useful when booting the vm, in brief they are:

1. path to find shell script to start the vm,
2. ways to login,
3. how to determine if virtualization is enabled.

[lucab]

One last comment, but I think it is generally fine. You can happily squash all your commits in a single one. I'm not a native speaker though, so you'd better wait for a quick check by somebody in the US.

[Bubblemelon]

The most recent commit changes the sdk-modifyingguide in the following ways:

1. Clarifies how to exit from container linux login prompt
2. The acronyms are now capitalized, i.e. VM, QEMU
3. The phrase "booted filesytem" is now "SDK chroot"
4. The word "login" is now "log in"
5. Doing is replaced with Running
6. Re-titled "Several ways to boot" to "Boot Options"
7. Rephrased the forget password sentence
8. Clarifies SSH log in
9. Absolute links are now relative links

[Bubblemelon]

@lucab I've squashed my commits

[Bubblemelon]

@bgilbert let me know if this is good for a merge :smiley:

[Bubblemelon]

cc: @ajeddeloh