device_path: add more convenience (part 1)

[phip1611]

This MR adds convenient methods to transform device paths easier to text. This should make it easier for devs to work with device paths (through convenient debug output).

The debug output of my example is sth like this:

for the first helper method (.to_node_string_pairs)

for the second helper method (.to_string)

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See the Rewriting History guide for help.
• Update the changelog (if necessary)

[phip1611]

@blitz Is this something that you think is helpful?

[blitz]

Should be useful for our internal loader usecase. 👍

[nicholasbishop]

The path will be different on aarch64, so might want to use an if cfg!(...) here

[phip1611]

right! At the first glance, I do not see where this difference comes from. Can you give me a pointer, please? I guess, its the way that xtask prepares the QEMU command.

[nicholasbishop]

I think it's just due to the edk2 firmware. Although we tend to refer to it as OVMF, OVMF is actually only for ia32/x86_64, while ArmVirtPkg is used on ARM.