QEMU-related enhancements

[zachriggle]

This refactors some of the QEMU-related code, adds documentation, and adds support for discovering the default QEMU_LD_PREFIX value.

QEMU_LD_PREFIX is an environment variable that determines where qemu user-mode emulation will attempt to resolve file paths if the "real" file doesn't exist. On Ubuntu, this is e.g. /etc/qemu-binfmt/arm, which is then generally symlinked to e.g. /usr/arm-linux-gnueabihf.

Previously, we hard-coded logic to expect /etc/qemu-binfmt. Other distros use other paths, and in fact vanilla (non-Ubuntu-maintained) QEMU uses a different path.

We now detect the path appropriately by checking the environment, and extract the compiled-into-QEMU default value by running e.g. qemu-arm --help. The help message contains a lines that look like:

Defaults:
QEMU_LD_PREFIX  = /etc/qemu-binfmt/arm
QEMU_STACK_SIZE = 8388608 byte

This goes all the way back to at least qemu 2.0.0, which is probably sufficient.